I had a JDM before and the issues I had were:
Radio - As above, you need a radio expander to get UK station. I don't know whether it can get all of them. It is quite cheap, looks like an extender for the aerial lead and you can fit in at the back of the HU.
SAT NAV - The one I had got UK / Europe disc, just need to work out what the Jap button means.
Battery - The JDM terminals on the car are smaller. The UK battery didn't fit as UK car got bigger terminals. You need to remember that. Not a problem getting 1 on the web but not sure you can get 1 from Halford.
Car Tax band - Mine was 52 plate, but it was import in 2007 so it was high tax band, I think 400+.
Dial - You don't need to change it, mine was the Jap one with the K blacked out and replaced with an M.
Limit - Correct me if I got this wrong but there is a speed limiter on the JDM car. I can't remember what it was but an HKS defender was installed to remove it. It was on the 1 I bought.
Version - it seems there are various different version over there but all UK car got Brembo?
When it goes through the SVA everything should have been done to make it legal on UK road anyway. i.e. fog light, Speedo.
I don't think owning a JDM is troublesome but I wish I knew some of the above before I got the car last time. I didn't help when the battery need changing and I tried to put a Halford one in only to find out the terminals are not right.
So this time I got a UK Zed.