Operating Theater: This is what we would refer to as an operating room or OR. The Brits just say theater. I guess it makes the idea of having an operation kind of fun...you could say "no, I can't have lunch tomorrow because I am going to the theater."
Boot: Trunk of the car.
Cattery: A kennel for cats.
Sellotape: Scotch tape (you should have seen me asking store clerks for scotch tape and then trying to describe it... grin and nod, grin and nod.)
999: comparable to 911 in the US.