Banks and ATMs they run will always give you better rates than the guys in the airport. Hotels usually provide a rate somewhere in between.
Since you are going to the UK don't get caught up in trying to do the conversion rate as you'll just get depressed. When you factor in VAT take that price in Pounds, say 1 Pound for a Coke out of a machine, and if it compares ok with what you might pay for something back home then get it. Just expect everything to cost double what you might pay back home and you won't be disapppointed. Have fun over there. I love the UK.
"If you can't delight in the misery of others then you don't deserve to be a college football fan."