Of course, the easiest and cheapest thing to do is convince a friend to loan you their unlocked GSM smartphone and buy a SIMcard when you arrive in the UK, and then Dublin.
If you travel to Canada and want cellular service for more than emergency use, then plan on purchasing a local SIMcard and perhaps using Telestial or another International virtual GSM network provider for inbound service.
Each has slightly different rate plans but it is still cheaper to buy a local SIMcard, carry a second (and possibly a third) GSM phone, and have inexpensive communications.
Assuming that you have an unlocked iPhone 4 or 4S or Android GSM (which is usually unlocked when sold), you can insert a local SIMcard and have local voice and data services at a fraction of the cost you would otherwise pay.