might have found my own answer.
Virgin prepaid mobile ... 10c a minute to mobiles in the UK

yep will be using skype when they are in an internet area and have their laptop with them.
To use skype to call a mobile is 26.7c per minute, which is cheap but just thought there might be something better
might as well save a penny if you can

I think that my lad took his Aust (Telstra) mobile with Int roam with him. That enabled him to call us to make contact with us from any where at exhorbitant rates. We could text/leave messages for him to call us as required when he logged on. When in a location for a while he would purchase a prepaid local simcard and we could call him on that number via Skype at Skype rates or he could call us via Skype from internet cafe. We tried to limit calls to and from Telstra Int roam to emergencies or to make contact for a Skype to Skype call. Takes a little bit of mucking around but in the end it is pretty low cost communication