T-Mobile has a special offer running on its Full Monty SIM plan
Plan is usually £21/month but if ordered online via the link above you get the first 12 months service for £16 per month.
2000 minutes - Any network, any time
Unlimited texts - Any network, any time
Unlimited data on your phone
WiFi access on the London Underground at no extra cost
Obviously check the terms and conditions before signing up, as with any contract.