I'm only assuming they'd have to withdraw from it out of competition concerns as with the purchase of BD they'd have a total monopoly on Belfast / Dublin to LHR flights - which surely would draw the attention of the regulators
A quick check of the BMI website has them flying to over 20 countries. You have an issue with them on 2 routes to 2 different countries. We also have no idea how much profit is attributable to those routes when compared to their entire network. Small fry I would say for a regulator. The bigger issue would be the total share of slots at LHR post merger compared to their nearest competitor. But lets face it there are much smarter people than us who would have crunched the numbers and have the responses ready the regulator. If they didn't then they wouldn't have wasted the time and effort on putting in a bid in the first place.