I do this and it works very well. Although I now save ££ and put them aside to transfer into a fee free credit card for spending there, if I see a rate I am happy with at any given time, I 'lock in' the rate by just buying some cash or putting money into my FairFX card. I've been doing this continuously for years and do so regardless of whether I have a trip booked, have just come back or am already there. It also removes the temptation to continuously convert when you look at prices in dollars which IMO shouldn't be done if you have already bought your currency, but that's another discussion