I think BMI were SNN's last real hope of retaining LHR. Even if the Government or Airport were to manage to buy LHR slots (which at the best of times is nigh on impossible, let alone with open skies round the corner) who would operate it?
Any carrier who did look at it would already have to have a base at either LHR or SNN to make it viable as setting up a new base would be costly.
EI are giving it up, BA & BMI don't want it. Their are plenty of airlines based at LHR but all the likely ones have been named? Who else would be interested in operating this route?
SNN only has one based option and that is FR - which would be an eyewatering sight at LHR but prob wouldn't happen as the landing charges would be 'outrageous' as FR regularly say.
And finally as Ryan2000 said the extra FR sectors wouldn't exactely encourange anyone onto the route.