http://o2roaming.co.uk/
35p = ?0.50 a minute, for incoming and outgoing calls, on contract and prepaid (contract with ITS stays at 28p receive)
Compared to most global SIMs own tariffs, and even using some independent callback with some single country SIMs*, this could be actually cheaper for calling mobiles. FL mobiles with CBW still cheaper though.
* eg Enlinea France mobile to UK mobile costs 69c, though Voipfone would be around 20p to 25p.