Harrier
Regardless of whether or not the Harrier would have any operational relevance to add to any current or future conflict it is a completely moot point. We do not have the pilots to fly them.
A shed full of Harriers would need to be maintained. That needs engineers. If they were pulled out of the shed and brought up to operational readiness that would require a shed load more engineers.
You would then need pilots. They would need training to fly an aircraft that most of them have never flown.
Those engineers and pilots would need to be drawn from current fleets. Which defeats the whole object.
The same argument holds true for Tornado, Jaguar, Phantom, TSR2 and any other historical type you can think of.
Take me as an example. I last flew a Jaguar in 2007. If the UK ever became desperate enough to need to bring them out of retirement, firstly, that would be a very bad world and secondly it would take me a good few months to get back up to speed and be useful. Thats aside from the fact I am no longer serving. If you took one of my colleagues who used to fly the Jaguar and is still serving, they are all either flying Typhoon, F35, helicopters or a desk.
BV