If you use BT's broadband service, you'll be able to subscribe to the "PlayPack," a monthly program that gives you unlimited access to over 100 games, for 3 months at no cost. That offer through BT can be found here. Oh, and OnLive play won't count toward your BT usage limit from now until January 2, 2012. It may start accruing after that, unless BT offers any other usage deals.
Additionally, if you visit OnLive at the Eurogamer Expo, you can get a free MicroConsole, and all new UK OnLive users can get their first game for £1. Regular prices for those services are £6.99 a month for the PlayPack and £39.99 per purchased game, should you like it enough to use it again.