Let's look at an example. It doesn't matter whether we are talking about £, $ or € - the figures are the same.
First, the bank will give you a lower exchange rate than they get on the money-market - say 0.5% lower - so you lose 5 per 1,000.
Then they charge you an exchange commission of say 2%, that's another 20 per 1,000. And then, if you want to transfer the money abroad, they'll charge you a currency transfer fee of, typically 25.
So you end up paying the bank 5 + 20 + 25 = 50 just to transfer 1,000 abroad - that's 5% - now that really is expensive - especially if you need to do it every month, or if you want to transfer a 100,000 lump sum payment.
It pays to look at other alternatives.
How Much Can You Save?