Buying the FTSE 100 Buys You 3 Banks, 2 Oil Majors, and a 45% Bet on 10 Stocks — That's Not Diversification, It's a Gamble

The FTSE 100's sector composition tells you everything. Financials, energy, and basic materials — banking, oil, and mining — dominate the index. Technology? Barely registers. The UK's genuine tech success stories — ARM Holdings (delisted in 2016), Sage, Darktrace (taken private) — are either gone from the public markets or too small to move the needle.

This isn't pedantry. It means that when you buy a FTSE 100 tracker:

• You are making a massive bet on oil prices. Shell and BP alone account for roughly 15% of the index. A sustained oil price fall — entirely possible given the global energy transition — hits your tracker hard.
• You are making a massive bet on UK interest rates. Banks (HSBC, Barclays, Lloyds, NatWest) are another 12-15%. Rate cuts squeeze their net interest margins.
• You have almost no exposure to what actually grows. The UK's vibrant fintech, biotech, and software sectors are either privately held or listed on AIM, where they sit outside the FTSE 100 entirely.

An active manager can simply decide to own less of Shell and more of, say, Games Workshop — a UK mid-cap that has returned over 15,000% since 2000. Your index fund cannot.

Market-cap weighting sounds neutral: each company gets the weight the market assigns it. In practice, it means you automatically buy more of whatever has gone up and less of whatever has fallen. You are systematically overweight overvalued stocks and underweight undervalued ones.

This reached its logical conclusion during the dot-com bubble. In early 2000, the S&P 500 was 35% technology stocks. Vodafone alone was 15% of the FTSE 100. Index investors rode those stocks all the way up — and all the way down. The index didn't protect you from the bubble. The index was the bubble.

Today, the same dynamic plays out more subtly. The "Magnificent Seven" US tech stocks drove the vast majority of S&P 500 returns in 2023-2024. A global tracker with 60%+ US exposure means your "diversified" portfolio lives and dies by seven companies in California. Is that what you signed up for?

An active manager, by contrast, can hold a genuinely equal-weighted portfolio. They can trim positions that have run too far. They can own 60 stocks at 1.5% each instead of 10 at 5% and 90 at 0.1%. That is actual diversification — not the statistical illusion sold by market-cap indices.

The FCA's own data on UK fund charges shows that many active funds are essentially "closet trackers" — charging active fees for portfolios that closely mirror the index. That's the worst of both worlds. But a genuinely high-conviction active fund with high active share is a different proposition entirely.

Here is where the SPIVA data — so damning for large-cap active managers — tells a different story. In less efficient markets, active managers do better. UK smaller companies, AIM stocks, and certain specialist sectors are genuinely under-researched. There are fewer analysts covering them. Information is less efficiently priced. This is the terrain where skilled stock-pickers can find genuine edges.

Consider the performance of active UK smaller companies funds over the last decade. Several have delivered annualised returns 3-5% ahead of the Numis Smaller Companies Index after fees. This isn't luck — it's the persistence of an information asymmetry that passive funds, by definition, cannot exploit. The index owns everything at market weight. The active manager owns only what their research says is undervalued.

Our deep-dive into Return on Equity shows how fundamental analysis can separate genuinely high-quality businesses from capital destroyers. That analysis — applied systematically — is exactly what good active managers do. The index can't distinguish between a company earning 25% on equity and one earning 5%.

For the ultimate comparison, look at what happened to Neil Woodford. His downfall wasn't active management — it was liquidity mismatch and an unlisted-stock strategy inside an open-ended fund. But the narrative "Woodford proves active management doesn't work" is lazy. It proves that bad active management destroys capital. Good active management — concentrated, research-driven, with aligned incentives — has a track record that the SPIVA headline numbers obscure.

An index fund never fires a CEO. It never votes against a remuneration report. It never sells a company because its strategy has deteriorated. It owns everything, forever, at exactly the weight the market assigns — including the zombies.

The FTSE 100 contains companies that have been shrinking for decades. Their market caps fall, so their weight in your tracker falls — but they are still in there, still burning your capital, still paying dividends they cannot afford. An active manager sells them. The index holds them until they drop out of the index — at which point they've already destroyed significant value.

This is the paradox of passive: you participate in the upside of every winner but also in the slow, grinding decline of every loser. Over short periods, this doesn't matter. Over decades, in indices with significant churn, the drag from index membership — from holding the walking dead — is real and measurable. Research published by Morningstar has documented how index reconstitution effects can create systematic drag, as stocks entering indices tend to be overvalued and those exiting are often at distressed prices.

Active management, at its best, is about refusal. Refusing to own the overvalued. Refusing to fund the incompetent. Refusing to sit passively while management teams destroy shareholder value. These refusals — when executed by genuinely skilled investors — create returns that no index can replicate.

As we explored in Free Cash Flow Explained, the ability to distinguish between genuine cash generation and accounting profit is one of the most powerful tools in an active manager's arsenal. The index owns both — the cash-generative compounders and the accounting fictions — at their respective market weights.

The smartest UK investors use both. A core passive allocation for cheap, broad market exposure — global trackers, developed-world ETFs — combined with active funds in areas where stock-picking genuinely adds value: UK smaller companies, emerging markets, specialist sectors.

A sensible approach for a £100,000 ISA:

• £50,000 in a low-cost global index tracker (0.12-0.20% fee)
• £25,000 in a UK equity income active fund with a proven long-term manager
• £15,000 in a UK smaller companies active fund
• £10,000 in an emerging markets or specialist active fund

This gives you the cost advantages of passive where markets are efficient (US large-cap, developed markets) and the information advantages of active where they are not (UK small-cap, specialist). The ISA allowance of £20,000 for 2026/27 gives you space for both approaches within the same tax wrapper.

The FCA's investment pathways framework for drawdown pensions — while imperfect — implicitly acknowledges that different market segments require different approaches. A one-size-fits-all passive strategy is cheap. It is also lazy.

For more on building a diversified portfolio, see our investing hub and our guide to dividend yield explained.

Let's put numbers on the concentration problem. Imagine two investors, each with £100,000 in their ISA:

Investor A buys a FTSE 100 tracker. By default, roughly £8,500 goes into Shell, £7,500 into AstraZeneca, £6,500 into HSBC — and so on down the list. Their portfolio's fate is overwhelmingly tied to oil prices, pharmaceutical pipelines, and bank net interest margins.

Investor B buys a global tracker for the core (£50,000), an active UK smaller companies fund (£25,000), and an active UK equity income fund (£25,000). Their UK exposure is deliberately tilted away from the mega-cap concentration. They own the same UK economy but through a fundamentally different lens.

The blended portfolio doesn't eliminate sector exposure — it diversifies it. That's the difference between concentration risk masquerading as diversification and genuine diversification through active tilts.

With the Bank of England's base rate at 3.75%, bank stocks have had a strong run as higher rates boosted net interest margins. But if rates continue to fall — as many expect — that tailwind reverses. The FTSE 100 tracker holder gets the full reversal. The active holder can trim bank exposure before the squeeze. That's not market timing. That's risk management.