A owns X Ltd (100% - and is also a Director)
A provides services to Y Ltd (which A owns 30% - and is also a Director)
X Ltd has other significant turnover not related to Y Ltd.
In principle I don't see an issue with A billing Y Ltd from X Ltd - ie. fee invoice only.
Higher rate aside for a moment, nor would there be a problem with A being only salaried at Y Ltd - especially as NI starts again.
The question is, is there in priciple anything legally applying if A 'bills' Y Ltd with a combination of salary and fee from X Ltd., given that tax strategy would be a part of this reasoning?