Using fancy dress
With such a rich history, Britain has a myriad of ways you can theme your private or corporate function.
 
Hysterically historical:

• Why don't we start with civilised life in the
  16th century:
  Roast bone chomping Henry VIII,
  William Shakespeare and all his plays and sonnets will give you plenty of theme party fodder (eg- Romeo and Juliet, Taming of the Shrew etc),
  Sir Francis Drake discovering the world and bringing it all back to England. Among other things he introduced were sherbet and the potato,
  Queen Elizabeth 1st - Rowan Atkinson's Black Adder springs to mind here too. (Bring on the Morris dancers and Whirling Dervishes!). A video library will be able to lend you the tapes or DVD for some excellent ideas on costumes and events of the day.
    
• 17th and 18th Centuries aren't ideal for the British theme. This era brings to mind an execution of Charles 1st, battles with the plague and the Great Fire of London... not a huge amount to celebrate. Also, the 18th Century makes us think more of France than England as they took the lead in fashion, art and music.
      
• 19th Century is almost completely the Victorian era for England and makes for a good strong theme. Think of corseted bustled skirts, mostly dark clothing (as we mourn the early death of Prince Albert), prudence (and fetishes just quietly) is the virtue of the day. The industrial revolution begins, cities grow too quickly and Charles Dickens is inspired! (Oliver, Pickwick Papers, Christmas Carol, Great Expectations etc). Also let's not forget the Romantic Authors Jane Austin and the Bronte sisters for this era too (Wuthering Heights, Jane Eyre, Emma, Pride and Prejudice...)
  Music Halls flourished throughout the country with their fabulous comic song and dance routines and cockney accents were enjoyed openly in the Theatres.
   
• Early 20th Century Things start moving much faster here so you can be more specific. Edwardian era for upper class is mutton leg sleeves, pearls, the mono-bosom and a bustle tapered to a hobble skirt (tight around the knees and flaring back out to the ground).
  We then totally move away from corsets into the roaring 1920's and silent movie era.
   
• 1940's World War II dominated and affected everything. Cloth was cut back to straight skirts, chignon for the hair, smart suits, red lipstick was pretty much the only makeup. Although set in France, the English TV series Allo Allo has been used to theme many successful parties. If your guests are familiar with this series, you can have some great fun with the French Resistance, English pilots and German soldiers.
   
• 1960's onwards What can we say? Austin Powers and the Beatles says it all. Britain was a world leader in popular music through this time and London was the hip place to be. From here on the British music scene continued to grow so whatever decade you take on will have plenty of great dance music to use.

Generic Theming ideas
So what else do we celebrate that is classically pommy?

• Seaside parties: Knotted handkerchiefs on our heads for down at the beach. "I do like to be beside the seaside". Think seagulls, boardwalks and ice-cream.
  Other Beach party ideas here
   
• Sport: They love soccer, rugby and cricket.
   
• Food: Fish and Chips or Pie with mushy peas and gravy make simple catering to fit the theme. What about Cornish pasties, Pork pies, Dumplings and custard, Steamed Suet pudding, Roast beef and Yorkshire pudding (similar to pancake batter); Trifle or Eton smash (mixture of crushed meringue, cream and berries). Listen to "Food Glorious Food" from the musical of Oliver for other ideas.