There's a lot of good info on that link Leah posted. Re the SVA (Single Vehicle Approval) system which applies to vehicles less than 10 years old, I've heard all sorts of horror stories about that. It's not something I've ever gotten involved in myself, as I'm a fan of older cars, but a lot of people have said something to the effect of it being bureaucracy gone mad. There's quite probably an element of inspectors misinterpreting the rules as well, and trying to apply more conditions than they should.
Whatever the age of the car, you'll need to get through the less-stringent annual MoT (Ministry of Transport) inspection anyway, just like U.K. cars. On that, I can definitely offer a few pointers.
To take some of the specific conversion issues from the PDF linked above:
In general, the test is looking to make sure:
Rear indicators are separate to the brake lights and the right colour (this is different to US cars)
Cars built specifically for the U.K. market have been fitted with amber rear turn signals by law since 1965, and it's surprising how many inspectors are around these days who don't seem to realize that anything else ever existed here.
It also appears to be something of a gray area on a single, privately imported car. The rules say that a car built after 1965 must have amber, but it seems that there
might be another rule which overrides that for a private import. I say might, because it's all rather vague.
In practice, you'll find some MoT inspectors who will stick to the 1965 rule and not budge. If you ask around the local area though, you'll find others who are more reasonable and will quite happily pass the car with its original combined red brake/turn signals. Older inspectors, and those who are themselves car enthusiass and/or deal with American cars regularly seem to be more flexible. That's my experience over the years anyway.
I've had a lot of post-1965 vehicles here and run them with the original red turn signals. In fact, I've had a few where somebody had tried to fit extra amber turn signals and made a real mess of the job, so I've just ripped out their additions and wired everything back the way Detroit intended.
Front indicators are the right colour and have repeaters on the side of the vehicle. This may require small indicators added to the side of the car and holes drilled in the wing as appropriate to accommodate them.
The correct white parking lights are present.
Again, since 1965 all U.K. cars have had to use amber front turn signals, but still needed white parking lights, so the combined parking/turn light which is still the norm in the U.S. can't be used here on new cars. The
really picky inspectors will grumble if you have amber parking lights, but in my estimation they're not quite so fussy about that as they are about red turn signals. So if you can find a "good" inspector who accepts the red signals, this shouldn't be a problem either.
Re the side repeaters, they're only required from 1986 onward, subject to the same provisos as above that a "good" inspector won't be too fussy. If you do want or need to have them though, you could avoid drilling extra holes by rewiring the existing side markers. (I'm not sure about newer vehicles, but for many years GM wired the side markers to act as signal repeaters anyway; Ford and Chrysler didn't though.)
A red rear fog light is fitted - US cars often don't have these.
Supposedly required from 1981 onward, but as above.
Headlights are pointing the right way - check before you bring the car in that this does not mean a costly set of new headlights!
Definitely needs to be changed, as the lights won't pass the anti-dazzle test otherwise, and even the "good" inspectors won't let that slip by. The older round and rectangular lights are easily replaced. The newer ones in weird shapes can be very expensive. You can, I understand, pass the MoT by just using beam deflectors stuck on the lights. (I expect the SVA wouldn't allow that though.)
Tyres match the advertised top speed of the vehicle. Tyres have a code (R, S or T rating) on them which matches a theoretical top speed. If the rating is lower than the manufactirers advertised top speed - you'll need new tyres (for the SVA anyway!)
Never heard of such a thing for the MoT. So long as they're not mismatched (e.g. crossply and radials on same axle) and in good condition, you'll pass.
The speedometer has Kilometer as well as Mile readings.
Not required for the MoT. Sounds like another daft thing in the new SVA scheme.
The basic road traffic law says that you must have a working speedometer which shows MPH. Curiously, the speedo is not even part of the annual MoT inspection, and you can pass even if it isn't working!
The right brake fluid level indicator is present on the dashboard or else the reservoir is clear.
Never heard of that in the MoT either. Must be another SVA rule.
The number plate meets UK standards (correct size, shape, lettering)
Oh boy...... We used to have problems with this in the past, as many of us run with U.S.-size 12 x 6" plates simply because nothing else fits the plate frames properly (and they look better!).
The registration law was actually amended a few years to take account of this, and it is now permissible to use regular 12x6" plates on imported vehicles where the normal U.K. size plates won't fit.
This would be only an MoT issue though, not SVA, as the latter would have to be carried out before getting U.K. plates.
Protrusion standards are met (ie removing long exhaust pipes, bull bars, mascots etc)
Again, I've never had any trouble in that respect on an MoT. Probably just the SVA again.
All in all, it sounds to me as though importing a less-than 10-year-old car now probably isn't worth the hassle. If it's over 10 years so you only have the MoT to worry about, go for it.