We no longer make aircraft exclusively for UK Forces. We buy them from other countries, predominately the USA.
If say, we ever get the F35, why do we need T&E of our own ? Not knowing the precise ins and outs, to the outsider (me) it looks a lot like taking a perfectly good aircraft, in use by other countries and their forces, and then picking fault with it "as it does not meet UK standards". And then spend millions to customize it so that it's late, over budget, unreliable and incompatible with every other variant.
This is strangely reminiscent of British Rail and its infrastructure in the 1980's which prevented any economies of scale in it's procurement.
Without the UK facility what exactly do we lose ?