Hi and welcome to the Forum.
Current EU rules permit you to bring your car; you will need to declare it to HMRC within 14 days of arrival and once you get that sorted (depending on the age and status of the car, you may have to pay some kind of tax), then get the car MOT safety tested, insured and taxed, you can then register it with DVLA and will be issued with a UK registration.
\the UK Gov website offers advice on the process; this link will take you there.
Unless the car is rare in the UK, you can probably get a second-hand speedometer from a car breakers yard; failing that, go to a dealer and order a new one in miles. That said, there is nothing forcing you to change the speedometer; we drove a left-hand drive car in the UK for 3 years - just had to remember what the speed limits translate as.
Hope this helps.
Cynic
Expat Team