I pay $0.17 a minute to forward to UK --
www.callcentric.com
Most Canadian prepaids charge a surcharge to call outside the region you are in, e.g. if you are in Montreal and want to call Toronto, you typically pay CN$0.10 a minute to make that call. HOWEVER, many plans do not surcharge beyond that to call the US. E.g. if you are in Montreal it costs the same to call either Toronto or Los Angeles.
That said, look at: AllFreeCalls.net. Free calls to the UK if you call their US number -- +1 712-858-8094.