Hi,
Interesting question if you had your UK registered vehicle, assuming it didn't already comply with the London LEZ and the German Green Badge Level, converted in Germany to there Green Level which would probably involve having a filter etc. fitted but also probably a lot cheaper than having it converted in the UK to meet the LEZ requirements, would it be acceptable to the LEZ.
Or perhaps looking at in another way would/does a German registered motorhome of the same make year and weight that wouldn't be allowed in the LEZ if it was a UK registered vehicle, be allowed free of charge into the LEZ if it has already been converted in Germany to there Green Badge level
Hope that's clear