I flew both my dogs directly into Halifax from Heathrow, on the same Air Canada flight as me and they came off the plane looking like they'd been to the dog spa, all relaxed and wagging tails.
I used airpets uk who were absolutely fantastic, it was quite expensive, but they dealt with everything and completely took the stress away. Everything went very smoothly, Heathrow have some pretty strict regs re size of crates and paper work and will not let your pet on the flight if something is wrong.
If you go onto their website to get a quote, give them the measurements for your dog and they will send you a detailed quote which will tell you exactly what size crate and what paperwork you will need, its up to you then if you decide to use their services or just use the info to get things done yourself.
There are quite a few companies who do this Petair were half the price of Airpets, but more expensive than others, but there service was superb.