I think all the purely for/against AD military arguments here have considerable merit.
However, these are politicians we are dealing with. To remove QRA, a token force already, would IMHO be politically unlikely as it would not go down well with the general public. At a time when lack of any kind of border control is causing huge unrest to further cut it, in any form, is an unlikely move by any of our leaders, whatever their political pursuasion. It is not about effectiveness, it is about perception. There is, I agree, no threat either. However, there is still a capability out there, which is actively demonstrated on a regular basis, do we just ignore it?
It is also another (small) cog in the counter terrorism machine, a hot topic now and for the forseeable future. Furthermore, AD is still an exportable 'business', especially to the Middle eastern countries and, therefore, a money spinner.
There is also our commitment to NATO in Europe.
I just think that pure capability arguments are irrelevant on their own. Since when has any Government ever made a decision based purely on practical need? They are sensitive to public mood (?)/media and, irrespective of the technical arguments, the UK public would vote to have it's military visible at home.
I think the bottom line here is that we need to effectively demonstrate our capability/intent to defend our own territories before we go a long way from home to defend others. Also, if you remove it altogether, it is gone forever. That strikes me as daft for an island nation.
I hope the SDR is balanced and looks properly into the future.