Your supposed to register it in the UK if it's here longer than 6 months I think. Not that many people do.
Visiting military is a completely different ballgame. The US may have a specific deal allowing them to continue using US plates. The paperwork can be a bit of a nightmare depending on what and how it's brought into the country so some may decide to keep US/German plates if they're allowed.
It may also depend on whether they live on base or not.
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