Well there is no catch, sign up one off fee £20 and you get three hundred odd txt, enough to run a little gsm unit
however it will give you free data for your ipad aswell!
so you get the same amount of data and sms every month , no catch
http://www.ovivomobile.com/
one thing to note, dont bother porting your number over, as the downside is if you get disconnected for some reason, they seem to be bad at that, where there communication is poor , and you may have over spent , but for general sms being free is fine, i did a little review search and found that to be a common problem