Well, I'm slightly embarrassed that someone launched an mvno in the UK without me noticing. As I knew their others, I look from time to time ...
It's on Orange UK, and the first virtual operator on there, beating Blyk by a couple of weeks.
The international call rates start at 6 pence a minute. There are other cheap int'l options on prepaid with almost all of the main networks, but the choice of the cheapest destinations is perhaps not quite as wide on all of them.
http://www.idtmobile.co.uk/rates/
Other IDT mobile operations - in Belgium and Netherlands their rates are similar to Lebara and Ortel
BE:
http://www.idtmobile.be/
DK:
http://www.globalmobile.dk/
NL:
http://www.idtmobile.nl/