I'm sure as Worried1 said they just have to declare an interest. I would phone them Jo! There's no way you should have to give up your right to compete if you have qualified, you can't help who the judge is! Phone BD and ask their advice x
It's really a no no at any type of Championships though, as has been said, they may well be able to swop the judges around to avoid your trainer judging your section.
If you're going to Patchetts then last year at some levels there were 2 sections as well as open and restricted so they may be able to change around quite easily or make sure she isn't judging your section. I had someone in the Elementary restricted there last year but with 2 sections it was just a case of making sure I didn't the judge the one she was in.
I normally have a rough idea of which of my pupils is aiming for AF's and Regionals etc and then if asked to judge I declare the levels I probably will have pupils in and ask not to be given those to judge.