At some points in the film text appears showing where they are. When it show that they are at the MOD building the spelling is "Defense" but it is actually spelt "Defence"
On the same theme : Austin refers to travelling in "an elevator". Not if he is British, he doesn't; we call them "lifts". He also refers to his car as a "Shaguar", a pun on the name of the car manufacturer Jaguar. The trouble is, he pronounces it 'Shag-WARR', based on the American pronunciation 'Jag-Warr'. If he was British he would pronounce it "Shag-U-Arr", because we say 'Jag-U-Arr'. Three syllables, not two.
Chi-Chi, the word defense with an s is printed on the MOD (ministry of defence, for anyone who doesnt know) building, which is /not/ believe it or not in America, its in London, and so /should/ have an English spelling instead of American.