Tom to flesh out a bit. You can indeed get you Levs here at a very good rate. I think the Post office are doing 2.67 or something shop around anything over 2.65 is good. Or take clean no rips or writting on notes not scottish and change in you hotel you might get a slightly better rate but the difference is pennies.
I would say that £400 is enough as we were there in Feb as a family of 4 and spent £500 although we didn't eat out a lot ( every lunch) and 1 night we certainly didn't hold back on the drinking.....have fun take your cash line card in case you run out...