AR15 M4 variants are not difficult to produce with in small modern CNC workshops ,a 5 axis milling machine can knock out most components , the UK has at least two rifle barrel manufactures and numerous companies with injection moulding capabilities , AR15`s are already being built in the UK for the civilian sporting market as well as some sect 5 ones for export .
Trust me, none of the bits are hard to produce - at all. Nor are they hard to produce consistently to a good spec. It wasn't that we couldn't do it for the SA80. The driver of the SA80's problems was having the design constantly value-engineered.
Forgotten Weapons has done some videos on this that are with a watch. Qualitatively, the earlier designs of what became the issue weapon were superior but the constant shaving of a little bit of cost here, the saving of a little bit of money there was the cause of much of the problem.
(I've included at the bottom the video relating to the XL60. The others in the series are worth a watch in terms of the history. They're both interesting and depressing.)
But, to come back to the thread topic, properly, there are plenty of companies in the UK that can make the components of a rifle - or an MBT/MICV, to address
@incendiarycutlery's point.
The question is whether we should go down that path, or whether we farm out whatever comes next to the Germans or the Belgians or the Canadians or the Americans or whomever. We're already a sub-80,000 'army'. Actually, we're a defence force.
To comment on
@QRK2's list: the SA80 was built here but a long time ago. BAE bought and then divested H&K as non-core. (I'm assuming that H&K got the SA80 make-it-work-properly work off the back of the BAE involvement/absorption of ROF/held intellectual property?)
The L7s built by Manroy - Manroy is now part of FN. Everything else on that list, as he notes, came from elsewhere.
That video: