In case anyone is interested, it seems Trading212 have launched a Cash ISA with 5.2% AER. Interest is paid daily for some reason - not sure what difference it makes to have it paid daily so maybe a bit gimmicky, but nevertheless at 5.2% might be the best it's gonna get for now if the BoE start cutting rates this year.
Could be useful if you don't want to lose your ISA allowance but don't want to invest it yet; and can still transfer to an S&S ISA later if need be.
https://www.trading212.com/isa?cash-isaI'm personally sticking with my building society Cash ISA at 5.1% so I can't offer any info on what the Trading212 account is like.