I've just received an email from Findmypast.co.uk. They are currently running a special promotion where they have credited some of their members' accounts with 25 free pay as you go credits credits. With a value of £3, this is a very generous offer, and there is absolutely no catch. You just need to log in to your account and use the credits within the next week.
Findmypast.co.uk have census records for 1841-1911, fully indexed birth records, millions of parish records, and military records for 1656-1994. In other words, plenty of ways to use up those 25 free credits!