From Chambers
PLEASE NOTE REPAIRS ARE NOT AFFECTED BY THIS NEW LAW
We are still able to sell major parts by mail order if they are part of an exchange/repair, so for example if we receive a cylinder/barrel/silencer etc. from a customer we can exchange it for a new cylinder/barrel/silencer etc.
We have marked on our site items that must be sent to dealers or incorporated into repairs.
https://www.gunspares.co.uk/News/
Exchange in my thinking is you send them your and they send you a new one and they keep your old one.