British food has a worse reputation than it deserves, and in any case, the culinary landscape has changed substantially. The UK is now genuinely one of the better places in the world to eat, particularly in London and other large cities.
The classics are worth trying: fish and chips from a proper chippy (eat them outside, from the paper, ideally by the sea); a full English breakfast with eggs, bacon, sausages, baked beans, grilled tomatoes and toast; a Sunday roast with Yorkshire pudding and all the trimmings; a Cornish pasty in Cornwall; Scotch eggs from a good deli. These are not ironic choices — they're genuinely good when made well.
Multicultural food is central to British eating. Chicken tikka masala is sometimes called the national dish — a fair assessment of how deeply South Asian cuisine has embedded itself. The UK has excellent Indian, Pakistani, Bangladeshi and Sri Lankan restaurants, particularly in cities like Birmingham, Bradford, Manchester and Leicester. London has every cuisine in the world at every price point.
Afternoon tea — finger sandwiches, scones with clotted cream and jam, small cakes, and a pot of tea — is a specifically British tradition worth doing at least once. It's expensive in grand hotel settings; more reasonable in tearooms and bakeries outside London.
Pubs are a social institution, not just a place to drink. A good British pub serves food — usually a straightforward menu of pies, stews, fish and chips, and sandwiches — alongside real ale, lager and cider. Cask-conditioned ale (served slightly warm, from a hand pump) is the traditional British beer and worth trying. Real ales vary enormously by region and brewery.
Whisky (Scotch) is the national drink of Scotland. Single malt whiskies vary dramatically by region — Islay malts are peaty and smoky; Highland and Speyside whiskies tend toward fruit and honey. A visit to a distillery is one of the best experiences Scotland offers.
Drinking culture is part of British social life, though attitudes are changing. The legal drinking age is 18. Drinking in the street is permitted in most places. Note that pubs in England and Wales must close by 11pm (or midnight with an extended licence); Scottish licensing laws allow later service.