AIUI, FSCS only covers cash, stocks were never covered by it. You get 85k per bank (though it includes anything you have in there separately), which I suspect is why T212 spreads your cash across multiple banks.
Regarding shares, it all seems to be detailed here:
tldr;
We pool clients’ assets together and hold them in segregated accounts with a custodian, completely separate from Trading 212’s own assets. This helps ensure that our clients’ assets are protected and that they can be easily identified and distinguished from our own.
So essentially, the only way you can lose stocks is if a) the company you hold the stock in goes under or b) fraudulent activity by 212 (i.e. not allocating the stocks to you in the ledger).
I don’t hold a lot of cash in 212 – only in between selling/buying – so any uninvested funds will be well below the £85K limit.
I’m very concerned and here’s why. I have a cash ISA in excess of £85K. When it was initially transferred to T212 I checked it was distributed so that no bank had in excess of £85K. On checking a week ago I discovered one bank now had in excess of £85K as the percentage distribution between the 3 banks had changed. I have no influence over how T212 deposits my cash ISA yet they seem to be happy to break the £85K guidance.