Hah what? Of course they're capable of building it. The UK is on the leading edge of nuclear power.
Rolls-Royce probably has some very nifty reactors, but they're naval reactors only, unless that's changed recently. And other than them I don't really see any British company at the forefront of reactor engineering and manufacturing.
Hah what? Surely you're not serious with regards to industry?
Yes. I am serious. Britain used to have big homegrown manufacturing primes that completely dominated the world. Not any more.
The shipbuilding industry is all but gone. The aerospace industry is not capable of making planes; yes it is a large industry that makes expensive stuff but gone are the glory days of multiple British primes that made entire aircraft, there is no prime that can truly make aircraft any more. BAE can sorta kinda make aircraft but only limited types (Hawk). British auto manufacturers no longer really exist, successful auto brands are either high-end niche products or foreign-owned marques. You have a decent rail network but no big rail manufacturers. Hi-tech industry (i.e. computing) on the scale of Japan/Korea/China/US does not exist.
Britain seems to do some fields very well. Things like healthcare, financial, education and media: i.e. not particularly heavy-industry associated. It is a rare British company that dominates, or is one of the dominant companies in a large manufacturing sector. Pharmaceuticals is a particularly good success story. Aero engines is another shining example. If I remember correctly, JCB is successful. The British manufacturing industry is also good at lots of niche products. But these companies seem to be the exception rather than the rule.
So I am completely serious in regards to British industry, and it's decline. Once upon a time it completely and utterly dominated the world, virtually every field had a British company sitting at the top. Now, not so much, which is a bit of a shame imo.