Proper (pronounced "Propa")--the best or for real, as in "I need a proper coffee. This one is too watered down."
Saddle--Seat, as in "The saddle on this bike is broken."
Choccy--slang abbreviation for chocolate, as in "Don't eat all those choccies at once!"
Telly--abbreviation for television, as in "Turn off the telly, we're late!"
Nibbles--appetizers
Fancy--Do you want, as in "Fancy a coffee or tea?"
Windscreen--windshield
Messing about--to behave in a silly way or to waste time, as in "I'm being serious and you are messing about!" or "They don't mess about, do they?" (this is almost interchangeable with "messing around" in the US)
Sweet--candy, as in "Fancy a sweet?" or "Give us a sweet."
Diary--day calendar, as in "My diary is really full this week" or "I need to check my diary before I commit."
Aluminium (pronounced ah-loom-in-e-um)--aluminum
Pegs--clothespins, as in "I need to buy a new peg bag."
To Let--for rent. These signs confused me at first because I thought they were signs for the toilet and that the I had fallen off. I'm more often looking for a public bathroom than I am an office to let.
Teats--bottle nipples
Spoilt--spoiled. They also say "learnt" here instead of "learned". It still looks strange but I'm sure I'll get used to it.
This is another spelling one. I guess here they spell it with one 'l' but in the US it is spelled enroll. It is the same with canceled vs cancelled and traveled vs travelled.
Plaster--adhesive bandage
Take the piss--joking around or making fun of someone, as in "Was she being serious or was she just taking the piss?" Warren thinks that people are saying "taking a piss" so there is some uncertainty around this one. But one thing is certain: it is not seen as crass here as it is in the US.
Speak--talk, as in "Speak to you soon" or "Let's speak about this in person". We definitely use the word "speak" in the US but it is a bit more formal. But here in the UK it is common in every situation to use "speak".