Lebara is currently offering free PAYG SIMs direct from its website (limit two per household), and if you port your number you get £5 free call credit within 72 hours of making your first top-up.
International calls are the focus on Lebara, with rates starting from 1p/min (including calls to Indian mobiles and landlines). Lebara to Lebara calls and texts are free. The first 10MB of daily data is free, and 15p/MB thereafter.
Lebara uses the Vodafone network, which gives them 99.7% UK coverage, and their customer service peeps are multilingual.