Thanks to the Music Industries association for this brilliant link. It even includes a guitar as an example.
You do have to enter your email address to see it but I don't think you are likely to be spammed since the site is primarily aimed at manufacturers. Nevertheless, for those wanting an executive summary, the point being made is that under the Trade Descriptions Act, you can label a product "made in Britian" if that is where the last substantial change took place. That includes assembly. In the guitar example, the product could be labelled made in Britain because the guitar was not playable until the work that was done in Britian had been done.
For orchestral stringed instruments, that means that you can quite legally import in the white (no pegs, tailpiece, bridge, soundpost or strings, and unvarnished) and get that work done in Britain and it counts quite legally as British made. Which is only what I have said many times before, but this says it better than I can.