Bigpants
You ask two very important questions and I have no idea of the facts that would enable them to be answered and I suspect all of the posters here are in the same position.
However this is PPRuNe so we can still offer opinions for what they are worth!
Dealing with value for money I don’t know how you judge the value of any defence expenditure until the period it is intended to cover is past! However a fair bit of the F35 programme money is being spent in the UK (we make back ends of all versions and the RR Lift System for the B) so that keeps people in high quality work plus they pay income tax etc. There has to be value in that. It also keeps our technology base at the leading edge of whatever is going on which has other potential advantages for the UK in the future. Personally I would not like to see us quit all that sort of activity.
As to the cost effectiveness of the B for the UK (assuming the carriers are there to enable our politicians to have their say around the world and they continue to want this) I am confident that the B is a much better choice than the C. Vertical landing is so much easier for the pilot. This makes operations safer with fewer training and currency demands. It also greatly expands the safe ship motion and poor viz operational envelope. Plus it requires only tiny fuel reserves compared with non VL operations. That is not just an opinion but fact. Many of the chaps who went to the Falklands were experienced catapult and arrested landing pilots and they all agreed that they would not have been able to operate in the conditions they found themselves in down there without VL.
In my experience it is hardly possible to overstate the reduction in stress on an approach if you can hover. For the approach to be successful (on land or at sea day or night) all you need to do is drift to a hover with the landing point in sight. You can then take out any lack of approach path accuracy with a bit of hover taxying. A minute in the hover is a very long time (ask any spectator bored by seeing nothing happening) hence the need for relatively tiny fuel reserves compared to normal fixed wing operations.
Going slightly off topic, I actually think the F35B spec (a supersonic and stealthy vertical lander) suits the possible UK carrier use more than it does the USMC for their primary expeditionary close grunt support role where I see the provision of supersonics and stealth as unnecessary – even unhelpful.