Trim Stab,
There would also be a requirement for coastal defence from smaller boats, anti submarine frigates and destroyers. Don't need to purchase hugely expensive bits of kit, just follow the examples given by Norway and Sweden. That way we might be able to afford sufficient numbers to actually defend our coast line.
Unfortunately this does ignore the fact that the large majority of UK supplies are delivered by sea, and by not having deep water vessels we will not be able to protect those supply lines. Cannibalism might then become an option/ necessity.
We will still require airborne assets to provide top cover, detection and offense.