Comair are forbidden to conduct long haul flights, they would require the express permission of BA in London
I have actually heard that BA mainline were actually ready to supply Comair with B777's about a year or two ago, to operate to the UK with their blessing, there just are not enough seats available on this route, but its the bi-lateral agreements and the slots at the UK side that are a real problem.
The other problem is the hesitant and ever over-cautious Comair board that are scared to take this risk.
The biggest problem in the near future and will be an ever increasing problem is the rate at which the pilots at Comair are now looking at the "sandy" pasture for better working conditions and salary, in a few months time their won't be any-one left to fly even the current aircraft schedules never mind the "dream" of international operations on B777's.