The presumption in all of this is that EI will cede some regional UK routes to RE. The benefit being EI don't have to deploy an A320 on a short thin route and RE get access to a new route without having to face the competition of EI. The EI brand name will also have a higher degree of recognition amongst the public. Finally, the reduction in capacity on the routes will most likely lead to an increase in frequencies which will aid EI in attracting connecting customers.
Cheers,
P2C