What makes you think BMI domestic links will be more secure if somebody other than BA buys BMI ? Ithink LH and its Alliance partners or VS will
have priorities other than LHR/MME or LHR/JER if they get hold of those
slots.
If BA get hold of BMI they could consolidate with their own domestic
services to give a good frequency e.g on LHR/EDI instaed of 8 BD / 10 BA , 12 or 13 BA. These routes have been shrinking anyway due to
increased competition fm other LON airports , LCCs etc and this will
probably continue especially if Govt continues with its Green Taxes which
have dis-proportionate affect on Domestics.
On the other hand if BMI does go to LH or VS , BA may be forced to reduce its Domestics to provide slots to shore up other markets .
You could end up with the scenario where VS or LH acquire BD and
abandon most domestics and BA then does the same and end up
with virtually no Domestics at LHR.