On the technical point, without doubt if and when it flies (sorry, for various reasons I can't say here what it is) it'll be under the Permit-To-Fly route, which is the UK's sub-ICAO category.
There are actually a huge range of different sub-types of permit, and so far as I know that's what virtually all historic aircraft in the UK fly under. It varies from the very stiff operating rules of, say, a Gnat down to a very relaxed limited envelope, simple rules, flown on an NPPL of a single-seat homebuilt. What I have to do is find the right balance, then convince both the owner and the relevant authorities that it's an acceptable one.
The main difference between the UK system and the US system (apart from the name) is that in the US you have to convince yourself that you've done things sensibly, and tell the FAA that you've done so. In the UK, you have to convince either the CAA or their "delegated authority" (most usually the PFA) which means a bit more paperwork - but the systems are more similar apart from that than most people think.
G