Answers for why the F-35...
OK I'll bite.
The F35 is currently the main new aircraft coming out of the US. Although thats a bit of a misnomer as huge amounts of the work (mainly production, but some design) are taking place all across the world in all of the countries which are purchasing it. That's part of the reason for the high costs. You need to give land A x amount of work, and whether what they produce is good or not, you have to take it. It often means you take it, throw it out and build it somewhere more reliable, which means you've just doubled the cost of that part, but the purchasing land A are happy.
You cant just buy new F/A-18's because there not in production anymore. Lockheed are building the F35, they're not going to undercut their new aircraft. So all you can get are second hand ones which mean reduced life compared to the new aircraft. In addition the F-35 will be a better aircraft than the older ones, if for no other reason then the advancements in Aerospace Engineering taking effect. Better materials, electronics, etc. all have a huge effect. Rebuilding the Harriers would not be the equivalent of the new F-35's because of these developments. Also trying to simply upgrade old designs with new parts/materials etc. is a huge cost. you may as well just design a new aircraft, the cost will be almost identical.
So why not, just start designing new UK built jets. Cost. It's that simple. Developing a new fighter Jet is a huge cost. With the F-35, the costs are split. It might not be as good as developing it yourself, but the cost is a LOT less (where talking billions of Dollars difference!). Look at the costs of the F-22. It is the best fighter out there by a mile, but the cost makes the F-35 look like you're buying a toy plane in comparison. Thats why despite it being the best, the majority of the US fleets will still be made up of the much cheaper and less capable F-35.
The F-35 wont be a bad aircraft, it will be at about the same level as the russian equivalent jets, about the same as the Eurofighter, a bit more capable then the (much cheaper) gryphon and should be ahead of the Indian and chinese native jets for at least a generation or two. But the cost might make it look like worse value for Money then a lot of the other options out there. But since when has value for money been important to the Military?