UK registered number plate in Europe but non UK font

The relevent German law, quoted below, states:

• must have their domestic license plates.

Note:

• license plates, not license plate number

So the real question is:

• are these licence plates that can be used in the UK?

Since you have already come to the conclusion that these plates are not (UK) domestic license plates, it would be wise to come to the same conclusion in other countries that have a similar rule as in Germany (which will probably be true in most).

A further look into the quoted convention / agreement below may give further details as to whether this a general rule or should be regulated country by country.

§ 21 FZV - Einzelnorm
Kennzeichen und Unterscheidungszeichen
(1) In einem anderen Staat zugelassene Kraftfahrzeuge müssen an der Vorder- und Rückseite ihre heimischen Kennzeichen führen, die Artikel 36 und Anhang 2 des Übereinkommens vom 8. November 1968 über den Straßenverkehr, soweit dieses Abkommen anwendbar ist, sonst Artikel 3 Abschnitt II Nummer 1 des Internationalen Abkommens vom 24. April 1926 über Kraftfahrzeugverkehr entsprechen müssen.
...

(1) Motor vehicles registered in another country must have their domestic license plates on the front and back, Article 36 and Appendix 2 of the Convention of 8 November 1968 on Road Traffic, insofar as this Agreement is applicable, otherwise Article 3 Section II Number 1 of the International Agreement of April 24, 1926 on motor vehicle traffic.
...