I know they do and there are other options besides - KLM, Lufty, Turkish, etc, BUT if EI can do it cheaply why not; after all, travelling through LHR, you have to interline, change terminals, go through security, pay extra taxes, etc. At the end of the day, most travellers go on a budget and the lower the proportion of that going on the trip there and back, the better.
A little swish marketing (call it the Pharoah-plane
), good deals on hotels etc. and it could work. Sure it's five hours on a 320, but if it's quicker and cheaper ...