By summing up the top 60 entries for UK billionares from The Times Rich List for 2024 (which is data from 2023), I get almost exactly 500 billion pounds in net worth (£499.55bn). According to Wikipedia, the UK had a total net worth of $15,972bn USD, which comes out to £12,298bn at the current exchange rate of 0.77 pounds / dollar.

That comes out to around 4.1% of the total wealth. However, the number from Wikipedia is from 2022, so this figure isn't going to be entirely accurate.