It doesn't have to be last in first out any more.
The company can generate its own criteria which if it doesn't breach any discrimination laws can be used. 'Seniority can be in the criteria if a partial reduction in a base occurs.
But there is nothing stopping them closing a base and everyone from that base being made redundant with years and years of seniority. Where as 80 miles down the road there is new base crewed by new joins. They don't have to chuck out the new joins and transfer the closing base crew to that base.
Then of course 1 year later establishing the base as a crew base again.