Keep in mind that I'm British so I grew up driving here, but I've had American cars in this country for years, both before I lived in the U.S. and through to today.
A lot of people seem to think that having a left-hand drive car in Britain is a problem, but I've never found it to be that big a deal, at least not in the places I've lived. There is the odd time when it's harder to see around another vehicle to pass, but on some roads (sweeping left-hand curves especially) being on the left side of the car actually affords a
better view, so it's swings and roundabouts in that respect.
We don't have much in the way of drive-thrus, toll booths, etc. in rural areas so that isn't really an issue. The times I have to use a ticket machine at a city parking garage or such like are so infrequent that they're not an annoyance. In the older vehicles with a front bench seat it's easy enough to just slide across to the passenger window anyway. Split seating makes it a little more awkward, but if worst comes to the worst you can just get out and walk around.
Even overall vehicle size has never been that big a problem to me. I've had things like the old Ford LTD station wagon, full-size '72 Pontiac sedan etc. and always been fine with them. Driving small country lanes you might have to hold back at a wider spot for passing other cars a little more, but that's really about all. Naturally it might be a little different for somebody living right in the middle of a congested city.
While you might have to adapt to driving with a passenger-eye view of the road, it really doesn't take long, and it's quite probably overriden by the fact you will be driving a car with which you're already familiar.
It appears that all we would have to do is tilt the headlights and do a few things to the mirrors. Not too expensive, and pretty easy! 
There are two rather different sets of formalities involved. Imported vehicles less than 10 years old are required to undergo the SVA (Single Vehicle Approval) inspection, which I understand can be an awkward process. As all my vehicles have been older, before SVA was introduced, and/or already in England, I do not have any firsthand experience of the SVA system. I think Dennis (the Menace) has some experience of this.
The other issue you will have is the general, annual MoT inspection which is required of all cars over 3 years old here, and which covers many of the legal requirements listed in the Construction & Use Regulations for vehicles.
The main bone of contention on American cars is usually lights. As you say, headlights will need to be replaced or converted for left-hand driving to avoid dazzling oncoming traffic. That's an essential.
All vehicles made for the U.K. market have been required to have amber turn signals front
and rear since 1965, so that's what the C&U Regulations and the MoT inspection manuals say. Some inspectors are willing to bend those rules a little when it's a single, private import, but many will raise an objection if you have red rear turn signals on a later car (or they might let it go on, say, a 1968 car, but not on a 1985 car). I've seen references in the past to some rules which are actually supposed to exempt personal imports from these requirements, but have never been able to track them down, so you've very much at the mercy of the MoT man. It's worth asking around to find a "good" one if you want to keep all your lighting as original as possible.
In a similar fashion, although the C&U Regs. demanded amber front signals here after a certain date, they still specify that parking lights should be white, so again if you get a strict inspector he might object to amber parking lights on the front. In my experience they're not quite so picky on that one.
Vehicles from 1980 onward are also required to have at least one rear fog light.
The MoT inspector I use a few miles from here is one of the "good" guys, and is happy to overlook trivial things like that on private imports (e.g. he passes my '87 Bronco II despite the red rear signals, amber parking lights, and no rear fog light). He takes the view that "It was built that way, everything works, so it passes." Unfortunately, others are what we call the Jobsworth type: "My book says....."
