It's an interesting idea that F-35 may one day be the RAF/RN's only fast jet. Not that fast, but it's a classification. If things work out well, F-35 could well serve its purpose as a small, multi-role war fighter, capable of self-escort up to a point and able to make a fist of most of the other associated, required roles. It should, that's the idea.
However, it isn't ever going to be a specialist air defence or air superiority (supremecy, or what ever next year's buzz word might be) fighter. It isn't designed with that in mind so it would be foolish to expect it to do that.
It may well come to that, but it is easy to see that we would end up with a very big and significant capability gap with just F-35 doing homeland defence. Unless the Government wants to face the worst possible scenario, a thread on PPRuNe titled "Scrapping the Typhoon was Bonkers", they might need to start thinking about the Typhoon replacement pretty soon. It does do the AD role rather well.
OK, a handful of people here will love to think that F-35 can do the pure AD job and that Typhoon is a dirty word. But, amongst its other, newer roles, it does do AD extremely well. Certainly better than F-35 will.