The UK has. and makes, our own nuke warheads. It's the Trident missiles we share with the USA.
The missiles are in a common pool in the US. Our subs go back and forward to the US to swap missiles when needed. The warheads we make in Berkshire and ferry up to Faslane by special train, where we fit them to the missiles.
The missiles, once on our subs, are independent of the US. And unlike the US system, our sub commanders can fire the missiles without any outside code etc. Hence our letter of last resort system.
Sky news article here from last year, when they visited Aldermaston. Note this bit : "The organisation has some 9,500 employees in total, including about 7,000 at Aldermaston, where the warhead is developed and its component parts are manufactured."
Inside Britain's largest nuclear weapons site - as scientists race to build a new warhead by the 2030s | UK News | Sky News
We make our own subs of course, and our own nuke powerplants for them. They are intertwined with the US nuke powered subs, but that is because we own some of the tech and they own some. Hence the AUKUS deal to supply Australia with Nuke powered subs involves us. Because we own part of the tech, and the US can't sell it without our involvement.
Re France. They have and make nuke capable jets. As do Sweden. The Eurofighter is not Nuke capable, but we still have some Tornadoes, and they are.
So yup, Europe and the UK are not defenseless.
:-)