I use Bitstamp and they can seem a bit flaky at times, the other day I moved some BTC from coinbase pro to them and it should have taken seconds, instead it took nearly an hour, saying that I do put my long hold stuff in there.
Never used Kraken, so can't comment,
For GBP conversions and daily trades I use Coinbase Pro, look at their Make/Take rates, they are at least as good and sometime better than BitStamp
For the wacky stuff I use Crypto.com
Depending on how much you are trading, all of these have a daily limit, but I personally have never hit it