The problem with the F-18 plan is that we would have to pay £1.6bil for the cats'n'traps, we would lose the X billions invested or committed in JSF and they have to pay for the aircraft so although "the aircraft" would be relatively cheap, the overall cost would be far from it - and we would have to find that money soon.
If you then factored in the loss to industry from the loss of JSF workshare it is even more expensive (although the ACA would be happy with the carrier mods).
Last we would be buying a 4th generation aircraft, which has its limitations, at the end of its growth life rather than a 5th generation aircraft at the start of its life (although I think F35B growth may be limited!)
F-35C is the best answer, but tbh if we can't afford it I really don't know what is - none of the answers are very good.