Interactive Investor charges a flat monthly subscription fee rather than a percentage of your wealth. For large portfolios, this model is vastly superior.
The Crossover Point
If you pay £11.99/mo (£143.88/yr) on ii, and Vanguard charges 0.15%, at what portfolio size does ii become cheaper?
£143.88 / 0.0015 = £95,920
Once your portfolio crosses roughly £96k, Vanguard becomes more expensive than ii. If you factor in ii's cheaper £4.99/mo "Investor Essentials" plan for balances under £50k, ii can be competitive even earlier.