It’s a question I get ask a lot. “Should I change my money here or wait until I get there?”
I wish I knew the 100% correct answer. But there isn’t one. Because it largely depends on the individual circumstances.
As a general rule if you are going to buy currency, it is almost always better to do so on arrival in the country you are visiting. In the UK in particular, there are all kinds of offers available with buy back guarantees and ‘no fee.’ But almost always they are at the expense of a non-favourable exchange rate.