Probably the cheapest and best option is to build more wind and not care too much if it increases curtailment.
Yes, all the things mentioned should be looked into and done when it makes financial sense but "wasting wind" is much less a thing to worry about than "burning gas", and I'd rather waste wind than waste money.
https://xlinks.co/morocco-uk-power-project/
Surely HVDC links between Scotland and England could be built?
And then there are pumped hydropower storage project like this one with a proposed storage capacity of 200 GWh and 1.5GW of power:
In the worst case, couldn't the excess power simply be used in electrolyzers to generate hydrogen? They may not be very efficient but it's better than throwing free energy away.
why would this be necessary when the entirety of Great Britain is one synchronous grid?
I suspect NIMBYism is a big part of the explanation. Airborne AC links are efficient but ugly. Underwater AC links are tolerated by Nimbies, but inefficient. So you end up with underwater HVDC links.
Building any new transmission line through densely-populated England is extremely expensive. Even if you can secure the necessary land and wayleaves, nobody wants them running near their house and spoiling the views, so significant segments have to run underground in tunnels, greatly increasing costs.
Besides, the UK is not that small when linking England and Scotland. The proposed Eastern Green Link 2 (EGL2) is 440 km long: there are many existing HVDC connections much shorter than that around the world!