Sim4travel currently has a promotional offer for the first 10,000 customers. Through this you can either get a £10 credit Sim for £8.49 (usually £15.99) and £20 credit for £15.99 (usually £25.99).
These sims give good rates for international calls whilst abroad and in many countries it's free to receive calls. A must have if you're travelling abroad this summer.
£10 URL - http://www.sim4travel.net/offers/cheapsim10
£20 URL - http://www.sim4travel.net/offers/cheapsim20
My first deal so be nice :)
£4 quidco too