Biff, here goes....!
Firstly the GT has a zero three quarter view, so it's a bit of a law unto itself for driving on the left in the UK. You can, of course, reverse it on the mirrors which is what I do for all of my cars - but that 3/4 blind spot is a big one on the GT. Angled T-Junctions (of which there are many) need handling with care.
Having driven many thousands of enjoyable miles in France in a right hand drive car - and driven the GT in the UK, I would say that it's not a massive problem to be in the wrong position.
To begin with it's terryfying, which leads to moderate fear, and then anxious calm. All in about 30 minutes. Dual carriageway or motorway driving is obviously really easy, country lanes are, as you've recognised, more challenging because you have to edge out slightly to get a view of the road ahead. Because the GT is armed with some serious acceleration, it's not difficult to enjoy on country roads. The trick is to lag back from the target car (!) a bit more than you normally would - and that gives you enough room/safety margin to edge out to take the look. You can also see up the nearside of cars - it's a novel and perfectly safe move.
The one thing that does happen is that you drift to the centre line a lot - I think because the hedge or pavement is closer than 'normal', you sort of shy away from it. Strangely, I don't do it when in France - I think it's being so used to being in the right seat perhaps. So it might not be an issue for you. I have the right hand mirror set so that I can see the centre line on the road and frequently glance at it to check my position.
It's worth stating the obvious and saying that there are no right hand drive GT's! So you have to keep it and you have to drive it over here! Why not import the GT first, see how you get on and then bring the others over? Leave them with a mate maybe until you're sorted out?
What are the other two cars? Left hand drive does NOT sell here in the UK (unless they are really special like the GT - and that's in tiny numbers). So I'd be really tempted to just bring the GT over (which you'll keep and love) and see how you get on.
Does this help? Neil.