Rakshasa,
Reasons for paying to be a Level 1 partner:
Getting in early has meant that over a hundred UK suppliers are now involved with building the aircraft and we have a very significant piece of the industrial participation. So when the US and other nations buy 3000+ airframes, the money the UK Treasury makes in tax on the sales of all those components will more than cover the UK money that's been invested so far and will also pay for the UK airplane buy. In simple terms, over the life of the program the UK government gets the airplanes for free, and should make a tidy profit (or if you're a cynic, you could say that there's plenty of margin for cost growth). You'll find it hard/impossible to get a deal like that if you buy any other airplane - F-18, Rafale, or Typhoon.
It also gets us a lot more access to the Program, at an earlier stage, than any of the other Partners.
Regards,
Single Seat, Single Engine, The Only Way To Fly