First of all Air Transat dont fly LHR-YTO, MAN or LGW i think are the two but might also fly from NCL.
Air Transat have been around for years, service on board is what you would expect from a bucket and spade.
The reason they are so cheap is they only fly half way back then shut down the engines and glide the rest of the way !!!!
http://en.wikipedia.org/wiki/Air_Transat_Flight_236