Canadian Affair

Canadian Affair is an agent for Air Transat and Thomas Cook, offering flights to every major and not so major Canadian destination — Toronto, Vancouver, Montréal, Ottawa, Calgary, Edmonton, Fredericton and Halifax. They fly from Gatwick, Heathrow, Manchester, Exter, Dublin, Shannon, Glasgow, Edinburgh, Newcastle and Birmingham.
In terms of customer service, I would suggest sticking to their online services. On the few occasions I’ve needed to ring their office to make changes to a booking or to try to find information prior to departure, the staff haven’t been particularly helpful. They’ll provide whatever you want — all for an extra fee.
Having said that, if you’re after cheap flights to Canada then don’t spend hours trawling Opodo, ebookers, Expedia, etc. You will not find cheaper flights anywhere else. As an example, I am flying to Canada in June with my two children and paying £763. They offer a £20 discount per person (each way) for instant booking payments too.
They also provide hotels and car hire at discounted prices.
So, cheap flights to Canada they do, customer service they don’t. Just make sure when you book those flights online that you won’t need to make any changes to them.