The mentioned above crypto related exchanges serves better tasks and have been very important for investors, holders of long term coins and the traders. We know the market is precisely stationed by seasons and hoping to buckled our shoes before the bull season kicks off. As for wallets, there's probably more preferable alternatives to consider in the space perhaps for clarification, it's better to make thorough findings regarding the corner point you're going.
You are right, we'll I don't know if OKX is officially used in the UK but I'm very familiar with it and I enjoyed using their P2P services before it got delisted for user in my country, well I think Bybit is another good exchange that could be used by traders and their P2P market is very okay, I currently used that since the P2P of Binance and OKX have been banned from users in my country.
But then I don't advise anyone to hold Cryptocurrency in exchanges especially if you're holding for a long term, maybe on could sell their assets using exchanges but holding is very risky except maybe the person is using it to trade or is buying a Cryptocurrency from an exchange.
Like you said, their are many alternatives asides exchanges to hold their assets all it just takes is a proper research on wallets that would be very suitable and well secured for holding assets since most exchanges are not very secured.