I was wrong. I just looked into Kraken's support page: https://support.kraken.com/hc/en-us/articles/360000381846-Fiat-currency-deposit-options-fees-minimums-and-processing-times-

And apparently, the deposit you're trying to make (with a 21£ fee) is not supposed to take more than 24 hours. Are you sure that you didn't get the money? Maybe you're just subject to the first-time fiat deposit 72 hours hold?