I think that even if there is a meal service going on in once cabin, you should use the lavs. in your own cabin.

If I find passengers using lavs in my section from another one - and that means First coming back to Business too (!) - I usually say something. Make it sound helpful ' You know there are more lavatories at the back ', if they're waiting.

Personally I only ever have a few problems as I try to plan my visits around the meals - you can usually hear the staff getting ready to send out the carts, and so I try and go before they get in the aisles.