Buying car in the UK to take to DE permanently.

This is standardized and quite possible within the EU. I don't know what the post-Brexit rules are but found this interesting article. Seems one moving to Germany permanently from outside the EU gets a break bringing their car that a current resident of Germany doesn't.
https://www.howtogermany.com/pages/import-car.html
“Importing a car from outside the EU
A car or other motor vehicle imported to Germany from outside the EU is normally subject to a 10% import duty and a 19% import value added tax. (The value-added tax on imports is called import turnover tax (Einfuhrumsatzsteuer).
However, if you are moving to Germany with the intent to become a full time resident you can, if you meet certain requirements, bring in your personal car (and household goods) free of duty and import turnover tax.
To escape the duty and tax you are be required to prove:
•    that you have actually given up your residence in the non-EU foreign country you are coming from
•    that you are establishing a new residence in Germany
•    that you have been residing outside Germany for at least 12 consecutive months (this can be waived if the reasons for your earlier return are beyond your control)”