Slatgrille, it's all about the bigger picture
BA departs LHR bound for LBA as scheduled. LBA RVRs are below limits so they hold.
The rotation is already behind schedule and if the aircraft was to get down it would perhaps suffer further delays on departure due to LVPs. Additionally, it may pick up a slot for a number of reasons, LHR is very busy after all.
The aircraft will have further rotations to complete that day plus there are two further LBA-LHR rotations that day. All with slots out out of/ in to LHR.
Therefore in my mind, the sensible option from BA ops point of view is to return the aircraft to LHR and maintain the OTP for the rest of that aircraft's schedule. After all, the A320fam fleet at BA doesn't seem to yield much in the way of flexibility.
It's not good for pax but the reality is they will be transferred onto other flights or accept other means of travel (mostly without a grumble).