It is ten years since I bought a motorhome in Germany, brought it directly to the UK and registered it in the UK. The only way to register a vehicle in the UK (or any country for that matter) is to go through the official procedure with the authorities and fill in the forms - you will then end up with a UK registration number which replaces the foreign one.
In my case, as I remember it, there was the usually inane form to fill out but first I had to have it inspected (standard MOT) and insured in the UK. As I recall, there is a set of import duty stipulations to meet which depend on how old the vehicle is and/or how long you have owned it. The latter one kicks in if it is a very recently built vehicle (if I am not mistaken). In the end my case was pretty straight forward but you will need to establish what applies to your case and take it from there. One thing to note is that the UK and USA are different from everywhere else and speedometers are graduated in miles. As a result, there is(was) a stipulation that the speedometer must show the speed in miles per hour (mph). A foreign vehicle other than from the USA is unlikely to be marked in miles (even the "dual" scale is unlikely to be there). Knowing that 30mph is 48km/h, 40mph is 64km/h, 50mph is 80km/h, 60mph is 96km/h and 70mph is 112km/h was all I needed in practice. I did not get the impression this graduation issue is heavily policed but I never exceeded the speed limits anywhere so it was never an issue.