What we have here is a whopper jr. If you get that with no sauce and no cheese and (if you have them) apple fries its 330 cal for the wholemeal..add a bottle of water and you are set! (I say have them there cause I live in the states but hope to be traveling back to the uk soon!)
EDIT: I just looked on the uk site for burger king, if you get the whopper jr no cheese, no mayonnaise and a small fry (you don't have apple fries) your total for the meal would be about 450 with a bottle of water or diet soda