It’s got be be equities via funds for c 5 years time horizon. You may be in the red for the first 2 years as markets move, but the c 4% divi reinvested starts to protect you from capital falls. After 5 years you are pretty immune from a crash as 5 years worth of reinvested divis is c 20%, and a crash is defined as a 20% fall.
Or put it in premium bonds and wait for the crash, you can get your money out in 1 month.
Once diversified and you start monitoring it you will be surprised at how much the capital values move around, 1 month some sectors are in demand, the next no one wants them. Just stay in the market.
Good luck, do it via the isa