I just got back from Stockholm, it's pretty damn awesome.
Expect £4 a pint (or half litre or whatever) at decent bars. Hard liquor will be even more expensive relative to here because access to it is controlled by the government, and reduced prices and special offers are illegal for most, if not all drinks. You won't see any 2 for 1 offers or anything like that.
Food is expensive, if you go for a starter and main with 1 beer at a mid range restaurant it'll be a minimum of £30 per person. Places like McDonalds are pretty cheap just like here, and there are plenty of newsagent-style places that sell sandwiches and drinks, again they are relatively more expensive but not break-the-bank extortionate.
If you're planning on doing some sightseeing or travelling further than walking distance, I highly recommend that you get a 'Stockholm Card', it gets you free transportation on the T-bana (underground train) and free entry to a load of museums and stuff like that:
http://www.stockholmtown.com/templates/StockholmsCard____9989.aspx?epslanguage=EN
My top tip for something to do is to take the ferry out into the archipelago, a return trip to Vaxholm will cost about 140 crowns, £12 or thereabout, to go further out to Sandhamn or Moja will cost about twice that but it's well worth it.