"Shaun of the Dead" and "Hot Fuzz" but if you're buying wait for the third in the cornetto trilogy to come out.
"Lock, Stock and Two Smoking Barrels" and "Snatch" from Guy Ritchie and "Layercake" from the guys that helped him make "Lock, Stock" and "Snatch"
"28 Days Later" I wouldn't watch "28 weeks later" cuz it's not in th same league. Opens well but doesn't keep the energy at the right level for the rest of the movie.
All the Monty Python movies, as mentioned, but especially "the Holy Grail" and "The Life of Brian"
"Human Traffic" a very light drug culture movie
"24 Hour Party People"
"This is England"
"ID" the best football hooligan movie I've ever seen.
"A Fish Called Wanda" with John Cleese, Michael Palin, Jamie Lee Curtis and Kevin Kline, and also "Fierce Creatures" with the same cast.
"Mean Machine," with Vinnie Jones, which was a remake of a Burt Reynolds movie and then remade into "the Longest Yard" with Adam Sandler, Chris Rock and, you guessed it, Burt Reynolds. The English version is much better though.
"Dog Soldiers" - werewolf movie
"Dead mans shoes" if you're going to watch any british movie watch this