I've got the Single Vehicle Assessment (SVA) appointment scheduled, which is a necessary inspection for all left-hand drive vehicles in the UK, and purportedly you're allowed to drive a foreign-registered vehicle within the UK from the port-of-arrival to the SVA test. I'm also paid-up for one year's auto insurance through Swindon, and have been reading up on the local "rules of the road", so hopefully the journey back will be as uneventful as possible!
As far as getting down to Southampton, it looks like there are frequent trains running between there and Reading. It's a snap to get to Reading from Maidenhead, so ideally I should be able to get down there in maybe 90 for £15 or so. We'll see how it goes!