YY to not rinsing the detergent off dishes - this gives me dodgy tummy
Sending cards for everything and making a huge deal about it.
Only ever filling someone's glass part way even though there's loads in the jug.
Not drinking very much liquid of any kind, especially at meals - are they not thirsty?
Brits not wearing shoes indoors - where I grew up, everyone wore shoes all time
hard placemats
Sorry
-Saying 'sorry' all the time - often this is when the Brit thinks the other person should say 'sorry'.
-Those announcements on platforms when the recorded voice says, 'I'm very sorry to announce ...'
-Using the clumsy phrase 'say sorry' rather than apologize. In the US, an entity at fault would (formally) apologize. 'Saying sorry' sounds childish and sarcastic to my non-Brit ears.
Insisting that 'British' spelling and pronunciation (which does vary across the UK) is the only correct way when, actually, both have changed considerably over time. Words like color and honor are Latin words and are correctly spelled without -u-. Check the Americanisms-bashing-thread for more examples.
And, despite those Americanism-bashing-threads, pretending that they don't understand flashlight, sidewalk, etc.
Routinely making unkind comments about the US President (even if he is an idiot).