General maintenance in terms of oil changes, plugs and servicing have proved to be fairly straight forward. There are inconveniences of course - I get frustrated with my local Lex Autocentre who do a reasonable service, because they ALWAYS forget to order the air filter in advance and then take up to 2-3 weeks to get it in after the service, (despite my helpful reminders beforehand!). That said, they provide a reasonable service for a reasonable price.
In general you will find that garages either freek out at the prospect of a non UK vehicle or they just take it in their stride.
I have settled on a combination of specialists and general places like Lex and I have found my local National Tyres location to be very helpful.
There is a place in Fleet, Hampshire called North American Motors that specialises in American cars. They have been useful for servicing and more specialist work that requires parts. They have access to an importer that gets them US car parts on a weekly basis.
When I smashed the rear bumper I found Finton Ellis - a young guy in Odiham, who enthuses about American Cars and does a fantastic job of bodywork. He is also linked into the American import network and has no problem with foreign cars. He did an excellent job of fixing the back end of my Explorer. The only problem was with my insurer who wanted me to go to my local bodyshop - who had no clue how to start - let alone get parts. A little persuasion on my part got the insurer to see sense and we all ended up happy.
For wheel balancing, all the normal specialists I went to could not handle a US vehicle, but Merrows in Farnham have the latest computer equipment which lists all cars on the planet it seems and they had no difficulty in sorting my Explorer out.
I have heard good things about Philip James, but never met him myself. He is a travelling mechanic who works on import cars. Private importers who have got his details from here tell me he is good. He covers the south of England.
Another place in Fleet called North Hants Tyres has been able to get me some parts which they import. They specialise in tyres, though I have not bought tyres from them to date.
I also bought the US maintenance manual for the car, which has been helpful for my own use, for listing parts and for lending to garages from time to time when they needed a bit more info.
The other top tip is to write down all the useful codes when you DO find the right plugs/filters etc so that your local garage can get them again next time - don't expect them to remember.
Finally, I have had a couple of recall notices from Ford US. The local UK Ford dealers have been useless and it was only recently that I discovered that my old chums at Mildenhall are registered to do free recall work by Ford US and other US manufacturers. They even offered to get the parts and instructions to my local Ford dealer with instructions to bill the labour back to Mildenhall who would handle all the paperwork. All at no cost to me!