Most UK Imports have to be valued through their respected service centres.
Mercs go to Shahnawaz and so on- They give an estimate- how much this car is worth. Custom duty is calculated according to that.
So if it's a fully loaded car, it will cost more.
JDM's are categorized in the engine capacity category i.e CC's. The higher they are, the more you pay. Simple.
I've seen UKDM Prius's bearing '10 reg even in Rajanah!
There are no problems with cooling or what- yes, they do have smaller radiators and are more configured for Cold climates.
As long as your vehicle is fit and it's running on coolant, it will be fine. Most German imports are from Uk... hehe.
Pros- Maintenance record, English language, not some next chichuaheuahfiejfauefoejofjaeipfe language as in Japanese.
Cons- Price and parts if not a Toyota or local brand. I mean, would you import a Vauxhall haha?
If you will be buying online, do check autotrader. They are more trustworthy, but a bit more expensive. Never trust Ebay or Gumtree. The latter is just a waste of time, if you do not know about cars and cannot physically go and check it out.
Make sure to pay a bit extra and get it's maintenance record and to make sure it ain't a CAT. D vehicle or something in that slang.
Sir @CrashD has some experience importing UKDM cars I assume. Maybe you can chip in sir? Pleasure.
Enjoy importing cars!