The Edinburgh route seems quite ambitious, due to the apparently near-insurmountable challenges of meeting channel tunnel safety and regulatory requirements.
Deutsch Bahn tried for years to introduce a route to London through the tunnel, but even they couldn't make it happen and gave up in the end. Eurostar's monopoly is well entrenched!
And there is, of course, a London to Edinburgh (and on to Aberdeen, etc) sleeper service already: https://www.sleeper.scot
The Orient Express (Venice Simplon-Orient Express) can "start" in London, but the restored antique train has never run in the channel tunnel. I've seen it done differently in most of the YouTube-hosted videos I've seen. The current method uses Eurostar to get to France and transfer to the antique train. Previous methods involved transfers on the British side from a fancy "Pullman" train.
One of the issues is that Eurotunnel requires a minimum train length, so that if a train is ever stopped in the tunnel, there will always be an emergency exit accessible without having to walk down the track. DB planned to meet the minimum length by joining two units together, but the problem is that there was no way for passengers to move between the front and rear units.
Deutsch Bahn tried for years to introduce a route to London through the tunnel, but even they couldn't make it happen and gave up in the end. Eurostar's monopoly is well entrenched!
And there is, of course, a London to Edinburgh (and on to Aberdeen, etc) sleeper service already: https://www.sleeper.scot