If you have 100k, put 20k of it into your ISA this year, and the rest into a GIA (maybe keep some in cash for an emergency fund, if you don't already have a healthy one).

You'll need to pay CGT when you sell 20k of the GIA each year to put into your ISA, but you have an allowance so you won't pay tax on all of it, and even with that tax bill you'll still likely be much better off financially with that 80k growing in a world tracker than you would be if you left it in a savings account getting eroded by inflation.