Investment platforms report a significant rise in Isa millionaires, fueled by investors heavily favoring UK assets. Hargreaves Lansdown noted a 70% increase in its Isa millionaires between December 2024 and December 2025, attributing this surge largely to strong UK equity performance. The FTSE 100 delivered a 24.7% total return in sterling last year, outpacing the S&P 500's 8.7% gain, according to Bloomberg. Hargreaves Lansdown highlighted that while some global funds exist, the top 20 holdings for its millionaires heavily featured UK equities like Lloyds, HSBC, and Shell.

Rival platform AJ Bell saw a 74% rise in its Isa millionaires, with Dan Coatsworth noting stocks dominated portfolios and the FTSE 100's success was key. However, experts warn this UK bias could pose risks. Concentration in one market is seen as potentially vulnerable to global shocks, especially with Middle East conflict and inflation concerns. Nimesh Shah of Blick Rothenberg cautioned against market concentration, pointing to recent UK market weakness. Analysts like Duncan Ferris of Freetrade link the FTSE's heavy weighting in financials and commodities to vulnerability to trade and commodity shocks, emphasizing diversification's importance.

While UK assets delivered strong returns in 2025, the market's international nature means diversification is crucial, as noted by Jane Sydenham of Rathbones and John Moore of RBC Brewin Dolphin, who stress opportunities exist beyond the UK. The trend reflects longstanding investor habits, though younger millionaires are emerging.