They are already debating to reduce the F35 procurement below 90 aircrafts. And if that comes at a cost of investment for a non US stealth aircraft. That won't be liked by the US.
138 F-35 is already too much for them in the first place. The F-35 procurement gameplan is quite simple. Boost the promised numbers before the start of the program in order to indicate the seriousness of all the partners. Once the program gains momentum, reduce numbers to more realistic levels.
This is especially the case where partners are far away from actual battlegrounds, the borders of Russia and China. So countries closer to those borders, like Japan, Korea and Australia, are increasing their numbers while the opposite is happening amongst partners farther away, like UK and Italy. Everyone else is planning on operating an adequate number of F-35 so there's no need to reduce the numbers further.
By that logic, even the US numbers should eventually decrease, as the USAF has hinted that before since LM is unable to reduce the cost of operation. Which is probably why the USG is willing to sell the aircraft in the ME to shore up numbers.
The way things are going, the UK may finish their F-35B procurement quite early. They currently plan to get 42 jets before 2023, 6 more by 2025, leaving only 20 odd jets. Which means there's not going to be any major fighter jet procurement for 10 years after the last of the 48 F-35s enter service. I don't know how you believe the UK cannot afford an R&D program during that time.
The UK will surely want a lot of things but realities are actually far away. They are not in any position to salvage more than 1 of the branches. Their land forces has already received their news. Now their air arm will be next. It seems that they want to save their navy the most. Their airforce and land force will become secondary support arms of their navy within a decade.
Their air force still needs a replacement for the Typhoon. So they either import that or make it themselves. And they are unlikely to import all of their requirement, hence at least one development program will happen.
Also, R&D costs will be split amongst three countries, and Sweden is very serious about getting a replacement for their Gripens even if Italy is not. And even Sweden doesn't have any planned procurements for 10 years after their last Gripen E is in. The Tempest program is inevitable.