Your Spanish visa will cost more than that soon [emoji23][emoji23][emoji23]. Your £20 is 1-2% of the amount your talking about.....hardly worth the effort of shopping around
I would say, though, that @PPF's Gatwick suggestion will guarantee you the worst rate anywhere.
Yep, agreed. A few quid either way not worth stressing about. I tend to just take a bit of cash to start with (usually have some kicking around at home) and then use ATMs locally.