Why can't the smaller places be closed and moved to the airfields?
In a large part they already have been.
Have a look at most MOBs now and you will see that they contain numerous 'londger' units that hav been crammed - many of whom have relocated the smaller units now closed. Most of those that remain are due to geography (such as the remote radar heads).
As for London apart from Main Building (which is thinning out rapidly) we only really have Northolt left into which lots of other organisations from smaller units (Uxbridge, Bently Priory, Mill Hill) are being crammed.
As for ZoomBot's Wittering 'REMFs who contribute so little to Defence' (yes, I'll bite). I'll let you go and tell an EOD operator from 5131(BD) Sqn that. I suggest you do that just after he or she has walked back from disarming an IED - I'm sure the comment would go down well.