Modern Elmo,
A little research before one posts can do wonders you know.
The UK MOD IS buying the F-35C, not the VSTOL B. The RAF doesn't buy anything.
ANY F-35 sales are equally good for the UK economy, the UK being the only tier one partner, fully involved in the design from the outset, and with a good chunk of each aircraft designed and built by UK industry.
As to why the F-35 and why not a developed Typhoon. You could just as easily ask the same question of the US Government, why not develop a multi role version of the F-18, F-16, F-15 or F-22?
So, apart from wanting a state of the art sensor fused LO strike aircraft with a decent air to air capability, why are the US Marine Corps bothering with all this strange vertical take off nonsense of which you speak, as so far they are the only folk who have said that they will buy the B variant?