Webacall is one of the cheapest, and correction - it's 6p to Tmobile, and 8p to other networks, but you are right there are cheaper ones like Voipwise, Voipcheap, or JustVoip (but those payment options are a problem). Maybe when i arrive in London the options will change (due to IP location).
http://webacall.com/rw/calc (and I should mention Webacall as per second billing, I'm not sure the Betamax companies offer that, do they?)
And your Betamax link isn't working, btw.