I've heard about Viber, and it seems to work well here. Does it work well in Scotland and England? Any trouble finding wi-fi to connect to?
We installed Viber when a friend of ours went on holiday to Brazil last year. It wasn't great, but I think that was mostly to do with the wi-fi at the Brazilian end, especially when our friend was 'up country' in Minas Gerais. Wi-fi here is quite widely available in coffee shops, pubs, McDonalds and so on. There is public wi-fi provision in some parts of central London and BT Wi-fi (formerly called BT Openzone) is widely available, but is very expensive. Outside of large cities you may have more difficulty, although even rural pubs often seem to have it. Of course any such connection will probably involve a DSL connection that may be heavily used and voice over IP applications need reasonable latency, so I wouldn't rely on it working everywhere.