I do not think it should come uinder the regulations.
My last McM is on order still, but i get them shipped to me directly from the US without any export regulations.
The same would go for the UK i assume.
As for the XLR wether it falls under ITAR due to the chassis construction and magwell, i can not say with 100% certainty.
As for if you ever need to get a ITAR item out of the US, you're best bet is to contact a company that does this for a living, they will take care of the whole paper mill required for a export lisence, for a few hundred dollars usually.
You would of course, have to procure you're own Import license in you're country, and send it over, along with identification papers and purchase order.
So it's certainly possible to get the chassis to the UK even if under ITAR regulations, but might be more costly and time consuming then you would like.
IF there is no regulations, and the producer still won't ship out of the country, witch is the case of several US companies.
There are a few companies that rents out US mail boxes to ship stuff through, witch would be a option.
There are also a few European importers that would make this whole thing go a whole lot more smoothly, will just be a bit more expensive.
Rimfiremagic.co.uk , is a UK company and they can get hold of a XLR chassis for you, I think this would be the easiest solution to you're problem.
Vertebrae.no do also carry them, but there is in many cases export regulations put on the European Importers from the US gov, to not re export these items, so wether they can help you or not i can't answer on you're behalf.