HUKD breaks the link, correct link here: http://www.productsandservices.bt.com/mobile/sim-only-deals/
Plan is currently discounted by £5 for the 4GB data, 500 minutes, unlimited text plan. Also includes unlimited BT wifi. Tethering is allowed.
Good deal for a BT broadband customers (...and actually you don't need to be the named person in the contract.. so if you've got friends/family on BT it can be sent there)
£35 cashback through quidco for sim only plans, which could bring the cost for the 12 months down to £85
BT mobile uses EE's 4G network, which has the best coverage in my area.