Strong opinions about queuing. Also, calling it queuing.
2. They are polite to people they hate and insult the people they like.
3. Asking them if they are okay, and they say "yeah, can't complain".
4. They ask you and everyone else in the office how you like your tea, and never ask again as they'll remember it forever.
5. When they say "Cheers" as an expression of gratitude most of the time. Oh and the 'mate' that comes next.
6. They use "quid" instead of pounds.
7. Use of telly instead of TV.
8. Use of the word 'mug' without going on to reference some kind of drink.
9. Make a cup of tea. BUT, in this order: Tea bag in, then milk and finally hot water. If they are British they will squirm.
10. When you catch their eye in public, they'll pretend to be looking for somebody else in every other direction.