There is a promotion at Reading T-Mobile store, (I am not sure if available at any other store), of £10 pm SIM under 12 months contract which gives you the following
300 inclusive minutes (normally 100 minutes)
300 texts (normally 100 texts)
A choice of a flexible booster (internet, unlimited landline, unlimited texts)
This is a great deal as most deals tie you in to 24 months, this is only 12 months.
I find their unlimited landline flexible booster most valuable of all as you can call a landline number for free any time of the day. If you choose this booster then your 300 minutes would be available for mobile minutes as the landline calls would not bite into your 300 minutes allowance, giving you even more call time.
I would definately recommend T-Mobile for customer service.