As my Jag is a Japanese import both the speedo and odometer read in kilometres. What would be the best way to change everything to miles for UK roads?

I was thinking about buying a good used cluster, putting that in, then getting the mileage corrected to read what it should be for my car. BUT would I also need to do something else for the odometer to continue recording distance in miles? Like a converter in the loom or something (as it's used to reading KM).

Does anyone have any better/simpler ideas, or has done this before? Thanks.

I think the only way is to change the entire instrument cluster. The signal processing for distance and speed is done in the cluster itself - the input to the cluster is simply the pulse signal from the ABS, that signal is processed and displayed in either miles or KM.

Going from memory, the only way to change the odometer reading is with both the factory diagnostic software connected, and then a one time use code is needed from the factory. Obviously, they want to limit the ability of odometers to be rolled back.

If you replaced the cluster/binnacle with one from a car with the speed transducer in the tranny (87 or earlier), the speedometer portion is different and won't work. You need the speedo out of an 88 or later. I had to get a new speedo when I converted my 87 to a diff mounted speed transducer.

I thought there was a button around the cluster that switches from English to metric

Probably in the owners manual

It might just pertain to fuel milage.

That button on my Jap import seems to do northing. I always wanted to know if was broken or not enabled ?

It only pertains to fuel consumption

The only way you can have odo and speedo in KMH or MPH is (as mentioned by @Jagboi64) by swapping the instrument cluster. The mileage displayed will be the ones that were recorded by that particular cluster....that much I do know from experience ...as for the changing the mileage itself--that I do not know...