There are various common signs you can use to recognise a counterfeit pound coin:
Example scans of various [counterfeit pound coins] are also available.
Note that the edge lettering on genuine pound coins has had its typeface changed several times, but it seems to be consistent within a year. What isn't consistent is the orientation of the lettering and its alignment with the faces.
Warning: Sub-sections 15(2) and 16(2) of the Forgery and Counterfeiting Act 1981 say:
It would appear that it's considered a "lawful excuse" to maintain possession of a counterfeit banknote (covered by the same statutory rules as coins) solely for the purpose of handing it over to the Police. See R v Wuyts  2 QB 476 and R v Sunman  Crim LR 569, for instance.
I suspect it's also permissible to destroy counterfeits yourself, as long as you're certain that's what they are. It's an offence under section 10 of the Coinage Act 1971 to destroy a genuine British coin, but this doesn't cover banknotes.
What about those coin pressing machines that press a 1p coin into a pretty shape? On the last one of those that I saw there was a little note saying how the law about destruction of coins no longer existed? --Adam
BJH21 has asked Cambridgeshire Constabulary what one should do in this situation. Their response is confidential, and not very useful. He asked some follow-up questions, but they got ignored.
If it's an offence to hold on to counterfeit coinage, presumably that means that if a shop detects a forged coin then you'd have to refuse to take it back?
Senji recently got two in the same day (3/6/4), his experience is [documented in his livejournal].