The owl from Luton at the frontier

Funny thing about British people: they correct my pronunciation with no hesitation. Last night I learned how to say "owl" and "frontier". Some weeks ago, when I tried to find out about the trains going to Luton, at least three people I'd never seen before (the lady who sold the tickets and two other people I asked for directions) corrected me immediately after I pronounced the "u" in Luton like the "u" in "nut". Well, two strangers corrected me and I kept saying it wrong. Maybe it means I'm stupid, but luckly there was the third stranger and I finnally understood it.