Even we (the UK) can't (or least couldn't at the time) get access or permission to modify the source code on the F-35 and we (as a minor but only Level-1 project partner) partially build it.
IIRC the only country that can is Israel and they got a specific exemption.
Honestly I find the way our government rolls over to the US frustrating at times, As the customer we should hold a firmer line - flip side is though that arms deals are often a proxy for general diplomacy so we take it in the shorts on specific deals for long term advantages - It's all in the (great) game.
The UK government boxed themselves into a corner by designing their new aircraft carrier to only handle STOVL aircraft. And the only such aircraft available is the F-35B. If they had designed their carrier with catapults like the French did then they would have had a couple other aircraft options, and thus more negotiating power.
A CATOBAR setup was included in the designs at one stage but later scrapped for cost reasons. Building a carrier with a ski-ramp is a lot cheaper.
The disadvantage is as you point out, severely limited types of aircraft for operations. The advantage of the cheaper design was that two carriers could be produced - France, with a similar economy and military to the UK size-wise, has only one fixed-wing carrier.
Two carriers is a game changer, as it allows year round operational readiness. For the Charles de Gaulle it spent ~900 days at sea in its first six years[0] - less than half the time.
Always having a carrier ready with "compromised" aircraft has many advantages over a lower availability with better aircraft.
A CATOBAR setup was included in the designs at one stage but later scrapped for cost reasons.
I don't believe it was ever really included in the design. EMALS was undeveloped at the time, and where would you get the steam for a conventional catapult, with no reactor and no boilers? And where were the compartments to physically put the (huge) catapults and arrestor gear? And even if EMALS was available where would you get the electrical power to run it (no reactor remember) when the (gas turbine) engines full output would be needed to head into the wind for launches?
It was a tickybox, that's all. Because BAe could never take the risk that the UK would chose Rafales or Super Hornets, they had to make the F35-B, which coincidentally they also have a finger in the pie of, the only option.
IIRC the only country that can is Israel and they got a specific exemption.
Honestly I find the way our government rolls over to the US frustrating at times, As the customer we should hold a firmer line - flip side is though that arms deals are often a proxy for general diplomacy so we take it in the shorts on specific deals for long term advantages - It's all in the (great) game.