That question will be answered differently depending on your circumstances, goals and in what you want to invest. Do you care about being able to transfer in and out in specie? Maybe not, if you are essentially saving for the time you cash out in retirement or for a specific big purchase.
Trading 212 is still a cheaper place to buy and hold Vanguard ETFs than the Vanguard platform itself. It is the cheaper place to buy and hold US stocks, even with 0.15% fx fee. ISAs were designed as Individual Savings Accounts. Some people use them as Trading accounts, even for US stocks, but that is not so attractive as it was since one can only hold GBP in an ISA and fx fees will mount.
Personally, I use ISA for long term savings and fill them with the least risky parts of my portfolio, mainly ETFs and US stocks I plan to hold on to for a long time.