All three! Why not? What's your preference in each? Being an uncivilized brute that is an American, I must confess to ignorance of the distinction.
Generally British beer is distinguished into three main groups (I'll talk about cider separately). Yeah, there a sub-groups within these three, but I'm going into general man-in-the-street definitions as if you'd walked into an English pub.
Bitter - Bitters are generally darker, hoppier beers served at or just slightly above room temperature. They tend to be less carbonated / fizzy than lagers. My favourite, personally, is Fuller's London Pride and yes, I also like Newcastle Brown (known in the Northeast as 'Dog' or 'Brown Dog'). IPAs (as Wals mentioned) are known as Indian Pale Ales and are lighter, cooler ales. It's important to remember that there are, literally, thousands of regional varieties of bitter. Well-known real ale brands include Hobgoblin, Brakspear, Worthingtons, Abbott, Badger... the list is endless. The nearest thing to it in America is the darker, hoppier beers you find in micro-breweries but these all tend to be chilled. English beer is seldom chilled.
Lager - a continental, not native, brew, lager is very popular. I prefer Czech, Belgian and German beers personally. American (domestic) lager is internationally renowned as being the worst in the world, I give you the hyper-chilled, tasteless piss that is Coors or Bud. I'm not talking the perfectly quaffable Sam Adams and it's ilk.
Stout - the 500lb gorilla here is Guinness, but there are other varieties of stout and porter. There are also hybrids like McCaffrey's.
Cider - madly popular at the moment as a summer drink, perry (i.e. pear cider) is also popular. I used to live in the West Country, the home of real cider (aka 'Scrumpy') which is usually poisonous, hallucinogenic apple gunk with bits of dead animal floating in it. It's fab.
I hope that helps, if you see what's available locally where you live that's from the UK let me know the brand and I'll give you a view.
Cheers
MC