They are expensive,my mates used to swear by 'Theoben' http://www.fishingmegastore.com/theoben ... 10300.html
Then you either need somewhere local to charge it or buy a divers bottle and the rest of the bits. Second hand Theobens might have had a coversion, where you can adjust an allen key at the back and the ftlb varies from 9 to 40+ . Over 12 you need a FAC of course.
Air Arms are also good. Think people used to like the 'carbine' . http://www.allcocksoutdoorstore.co.uk/s ... lnut-.177/
Your best bet is to get along to a Game Fair, there are always good people to talk to and bargains to be had in the last hour of the day.
And after all that said I still use an old German Spring Rifle with an OX spring and PTFE coated/tuned piston. Which of course is 11.9 ftlb.