As long as there is a Beech 1900 certified in a EASA State then the CAA cannot deny UK certification. I think Net Jets has one on the Portuguese Registry so putting one on the G from the CS (or indeed any other EASA State) would not be a problem. What would be more difficult would be to put a second hand N reg model on the G because ALL non standard Mods (either STC'd or not) have to be formally approved by EASA. (The CAA normally aprooved FAA STC's but it is not their responsibility now)
The reason why none on the G is that historically the CAA had an issue with it and nobody has tried since EASA took over.