Well I'm British. The first thing that comes to mind is the traditional American stereotype of we're all posh bastards who drink tea and have a stick rammed up our arses at childbirth. The fact is Britain is a land of stereotypes, seriously. Check out this action.
The first stereotype that comes to mind is North Vs. South. To the South Northerners are violent, drunken, idiotic, incoherent and chronically lazy. To the North Southerners are snobby, elitist, arrogant, and patronising.
Then there's the various regional stereotypes beyond that.
Cockneys, from London, are considered to be wheeler dealer shady types. Geordies, from Newscastle are considered to be Northerners x 10. The Scouse, from Liverpool have a reputation for nicking anything not nailed down. Essex Girls (from Rikki's home county of Essex) have a reputation for drinking a lot and shagging anything that moves, people from the Home Counties (those counties that border London) are considered to be snobby, arrogant arses even by Southerner standards, here in Dorset we're seen as being stuck in the 1950s. I could go on.
In terms of popular culture the biggest modern stereotype is the Chavs. Chavs are a phenomenon that has only arrived in the last couple of years. Chavs are basically morons, who think they're great. They have no money, and no instinct to work, but get drunk a lot, try to pick fights (though only when surrounded by other Chavs. Chavs, like Wolves, are cowards alone and hard nuts when there's 50 other Chavs backing them up) wear jewellery in a 'bling bling' sort of way, listen to faecese music and are generally the scum of the earth.
You can read more about Chavs are
http://www.chavscum.co.uk