Air Transat operates a good range of services to various Canadian destinations from across the UK. This year, they seem to be offering:
Birmingham - Toronto
Edinburgh - Toronto
Exeter - Toronto
Glasgow - Calgary
Glasgow - Toronto
Glasgow - Vancouver
London Gatwick - Calgary
London Gatwick - Edmonton
London Gatwick - Halifax
London Gatwick - Montreal
London Gatwick - Ottawa
London Gatwick - Toronto
London Gatwick - Vancouver
London Heathrow - Toronto
Manchester - Calgary
Manchester - Montreal
Manchester - Toronto
Manchester - Vancouver
Newcastle - Toronto
Flights seem to be operated by either Air Transat A310s or Thomas Cook A330s. Air Transat has its own website, and uses the Canadian Affair brand.
Iceland Express Another option mentioned is the route via Iceland. There are direct advertised connections on the London Gatwick - Reykjavik - Winnipeg route. In addition, Iceland Express also flies to Reykjavik from Birmingham and London Stansted.
Icelandair Go from Glasgow, Manchester or London Heathrow to Reykjavik, then on to Halifax or Toronto. If you're prepared to go via the USA, then Icelandair's Minneapolis or Seattle services could also be gateways to Canada.