The sticker price of the F-35B isn't the real cost to the UK as it manufactures a fairly large part of it. The sticker price of the F-18E/F is what would have to be paid plus the cost of a number of T-45s for training plus the cost of replacing the results of a steady stream of airframe losses during normal use.

The expectation is that if the new carriers were ever full of F-35s then a fair number would have USMC markings.