Something wrong here. Given that you're entitled to visit the EU for 180 days *without* a visa on a UK passport, you shouldn't need to provide a proof of residency.
Happens all over the world, though. Air NZ check-in in AKL wanted to see proof of residency for DE (have dual UK/NZ citizenship) on an AKL-LAX-FRA routing, showed it, he said "I can't read it, it's in German - what does it say?" "It says that I have residency" " Oh, OK then..."
You can't make this sh1t up....