How about this?
A soft Brexit with soft borders
- Britain exits EU, no freedom of movement, no access to single market.
- Scotland becomes independent/further devolved and joins the EU. It has access to single market and FOM. It also has a soft border with the rest of the UK for both goods and people from Scotland and rest of UK.
- Northern Ireland has a similar arrangement, perhaps more likely to be further devolution rather than independence.
So then firms with an office in Scotland/NI can export/import freely into EU. EU can export/import freely into Scotland/NI. Scotland/NI can export/import freely into rest of UK. Physical things just need to go via Scotland/NI (so customs etc goes there, plus maybe to the soft borders). So, for example a lot of financial services sector would move to Scotland/NI as passporting would still be valid from there, and the firms could still have linked London offices. It would take some of the heat/pressure out of the South/London, but wouldn't involve the same losses as people decamping wholesale to Frankfurt/Paris etc. Attractive as same time zone, same language etc as London (so good for e.g. US firms) and many financial firms already have small regional offices in e.g. Edinburgh/Belfast, so they wouldn't be starting from zero.
Scotland and NI have freedom of movement. But England/wales/Channels Islands don't. All still have relatively straightforward access to the single market.
The British Isles becomes less London-centric, more balanced as some firms relocate. Relocation also happens to areas other than NI/Scotland to e.g. north of England as it's closer to Scotland and easier to physically get goods there, also ports like Liverpool that face Ireland see a resurgence.
Existing EU migrants can decamp to Scotland/NI and settle there (Scotland actively needs immigration). After a set period/process/exam they can become citizens there and from there can move to the rest of the UK if they so wish, but this is a lengthy process that involves time and commitment. Similarly people from the rest of the UK moving to Scotland/NI- after a set period of time/process you can become an EU citizen and from there move to Europe, but again, it's a lengthy process involving time and commitment.
There would need to be regulation of what actually having a base in Scotland/NI constitutes- I.e it can't just be a PO and someone to divert calls.