Insert the word 'bloody' into sentences at seemingly random points. e.g. "What a bloody good idea"
Drinks pints of warm bitter. Always complain if served beer even slightly below room temperature. If aiming for the 'from the Midlands' effect, also complain if served under a full pint, in other words, if your beer has a head :-)
*never* wear a baseball cap. If you do you'll either a. Look like Willaim Hague, b. Look like a really sad tourist, or c. Even worse, betray your lack of Britishness and be branded "yank" (shock horror gasp)
Follow (and understand) both cricket and football (football optional for lasses) whilst all the time knowing full well England doesn't have a hope in Hell
Drink far too much and (chaps only) cultivate a beer belly, to which you refer as "your spare beer barrel"
Talk about the weather, even though it's been the same for the past two months
Grumble noisily to the people around you, but never to anyone in charge who might be able to do something about the problem
Insist that 'class isn't important', yet become highly irrate when Tony Blair (boo, hiss) announces "soon we will all be Middle Class"
Profess to prefer tea to coffee, indeed drink the stuff morning noon and night
Watch both 'Corrie' (Coronation Street) and Eastenders, and be able to recognise characters from 1964
Impress foreigners by your huge ability to consume alcohol: and your lack of concern when you subsequently vomit. Thanks go to MIKE ALLCOCK for this one
shout 'waaay' whenever someone (including close friends) drops something, spills something, falls over etc.
Detest the french. Detest their food, their personal hygine habits etc. The correct opinion is that "France would be nice, if it wasn't full of the French". Should anyone point out that they've been our allies for the past fifty years, the correct response is "Yes, but they've been our enemies for the past two thousand years".
Not that I'm inciting racial hatred or anything. 
Couldn't organise a piss-up in a brewery
Righty ho then chaps
cheers! NOTE: Should be used in the following circustances: As a toast (usually when drinking beer) and Instead of 'thanks' Thanks go to DOUGLAS B. KILLINGS, owner of the The End of The Phoenix site (see the links)
wanker! NOTE: literal meaning is 'one who masturbates', however is also used to mean simply 'idiot', as in 'learn to drive you wanker!' thanks again to DOUGLAS B. KILLINGS
Cripes! As a general exclamation of surprise/alarm/dismay. Simliar usage as Crumbs! Thanks to JOHN BICKTERTON
If aiming for the 'I just stepped out an Enid Blyton book' effect then also use (again, seemingly at random):
Jolly hockey sticks!
Wizard!
absolutely smashing
Whizzo! Lashings of ginger pop!
Come on Timmy!
God save the King!
"Let's all go away together for three weeks without our parents, even though Anne's only nine and even Julian's only thirteen"
"Come and live with me Noddy", "alright then Big Ears"
