It sounds to me as if the aircraft grounded in Moscow is an act of retaliation by the Russians. I can't believe that BA would choose to check it there rather than bring it back to base. After all, it's been flying around for a few weeks since this substance was on board, one more sector wouldn't have been a problem.
If they had returned it to the UK whilst being aware there was a potential problem they would be opening themselves up to all kinds of issues, hence the reason it remains in Moscow for tests. Flying the plane around exposing people to potential risk when you are ignorant of that is entirely different.