Truphone isn't your best SIM for the UK, it is handy for someone who splits their time between the US and UK. For example, my best friend is An American ex pat who lived in the UK for ten years (until the summer). He now lives in Oz. he is an occasional cell phone user. He only makes a couple of calls a month and receives six or seven calls a month when he isn't travelling. He is made for Truphone. He doesn't get robbed in the three places he goes and can simply use the phone like he is at home regardless of where he is.
If you are a heavy Truphone user and want to use it as yor contract phone, look at their business packs. I recommended it to a British performer appearing in the US and going back and forth. I can't mention names, but he/she is making over 20 grand per episode is not trying to be completely el cheapo, but was sick of being gouged. And loved having his/her US and UK mobile on one SIM. Paying $300 a month for a simple solution that worked fine in both countries and giving him/her data, phone, SMS that worked in both countries without hassle.
I use a Truphone for Oz because their prepaids expire so quickly. When I am in Oz, I stay at my friends and make most of my calls over wifi. I find Australian prepaids complex with flagfalls, and the fiction that they will "sell you $250 for $50" which really means you'll get 250 credits reduced by a complex formula.
Truphone is also helpful as a spare US phone which gives me access to TMobile's network, for foreign guests, etcs. I've rarely used it in the UK because my UK SIMs never seem to expire.