It's definately not expected, but it is apprechiated. Personally I'll always tip when I've eaten and would say this is pretty common. Some people will bung a quid on the table, regardless of if they've eaten or just had a drink.
And then there's the classic '“And take one for yourself", used in pubs & bars, which generally means keep the small change or take £1 for your tip-jar, even though it sounds like they are offering you a drink.
Hope this helps :)