Perverse logic
There is a possible perverse logic. That is:
We're going to spend around £8bn of tranche 3 Typhoon, and £9bn on JSF.
If you go for Typhoon M, you could spend an extra £3 - 4 bn navalising the 80 or so tranche 3 aircraft (plus fit cats and traps to the 2 CVF).. and still save £5bn. You end up with a one type airforce, but fantastically expensive aircraft.
BUT - this has been done to death. There are a load of other factors - including the work you'd need to do on the airframe, CTOL training regimes, UK earnings from the JSF programme etc etc etc. Use the search function!
I think we've concluded time and again that it's JSF or nothing.