Any company that holds or manages money is required to keep client money/assets seperately in case they go bankrupt or withdraw from the market, any client assets would either being returned or transfered to a different fund/platform/company.
The main risk is if the company is less than reputable there is a risk they don’t do this, you can see this in the various Ponzi schemes over the years, most recently FTX is good example of a company comingling funds/assets. It is fraud and carries a heavy penalty for those who orchestrate it, I think its unlikely you would be affected by this if you are using a well known brokerage/firm/fund but I guess there is always that risk. Fingers crossed Vanguard isn’t a massive Ponzi!!!! >.<
But yes provided the company has followed correct regulations customer money is protected and separate from the company, the company doesn’t own their clients shares or assets therefore any bankruptcy shouldnt hit your pot. (although there is always risk of fraud), a good check is to ensure they are registered with the Financial Conduct Authority, but also do a little research on how established the company is, how new to the market etc.. You can always split your pot up amongst 2-3 different companies if needed as well:
https://www.fca.org.uk/firms/client-money-assets I dont think any pensions or investments are covered by the £85k protection anyway regardless of whether they sit with a bank, in an ISA or anywhere else, I think the 85k only covers cash accounts like savings/current accounts.
In short I dont worry about it at all, I think if I had significant amounts built up (still early in journey) I might consider splitting it between 2 platforms but its not something I actively think about.
(fyi this is based on not having more than £85k of cash sitting around, if I wanted/needed more than that in cash I would definitely split it between different banks)