When travelling to UK & Europe I spent some time checking the mobile phone charges to evaluate the costs as our Australian prepaid sim cards do not roam overseas. In summary I found that for a husband and wife travelling and 3 travelling partners and with kids back home in Australia the following sim cards were good value for different situations :-

1) Best all round value - T-Mobile for cheap calls in UK and cheap incoming & outgoing calls while roaming in Europe
2) Great value for calling home & in UK - World Mobile. Note does not roam outside UK but calls are super cheap to Oz. Also you need a prefix of 1 for all calls overseas (but not sms)
Both these sim cards are available on the ebay Uk site and were posted to my hotel in lieu of posting to Australia.
3) for cheap incoming calls in Europe (no charges at all) I used a One Roam sim card. This is a world roaming sim card. They had good rates to most of the world but as of last year were not available on ebay. Also the sim card does not have an expiry date so it can be used on the next trip