Ales are classified here:
http://www.camra.org.uk/different-styles
John Smith's beer isn't 'ale', 'bitter' or anything else. It's just tasteless, chemical muck.
If you really want to learn about different British beers try . . .
Young's Double Chocolate Stout. (It's sold by Tesco throughout the year and by Morrison's around Christmas)
Shepherd Neame's Porter (which is sold by Sainsbury's as their own-brand London Porter and by Asda as their Extra Special Whitechapel Porter).
Asda's Extra Special Golden Ale (also from Shepherd Neame).
Asda's Extra Special Mainbrace India Pale Ale (Shepherd Neame again)
Brakspear Bitter (sold in Waitrose)
Thwaite's Dark Mild Ale. (Unusually good for canned beer, from Morrison's, Tesco and Sainsbury's)
Those beers are definitely NOT the very best British beers available. (There are many better ones available on draught in pubs). However they're all 'highly quaffable' and easily obtained sin supermarkets. Give them all a try to find out what style suits you best!