The reason it was ABZ and EDI ,is that it was those routes where overlap occurred between BMI and BA. BA was already the sole operator on GLA so the takeover of BMI by BA did not change the competitive position.
IF SRB is so keen to serve GLA as well he needs to fork out to acquire the slots or substitute GLA for other VS routes .He wants the slots given to him.
The spare slots VS had themselves were used for MAN which was obviously preferred over GLA.
There's no chance of NCL , BFS etc unless VS can get slots FOC and even then they would probably use the slots elsewhere unless a condition of being given them was they had to used on BFS or NCL.