I will mention EC261 on claim form as advised in the wiki. Any guidance on whether I should state on the form that my expectation is 300 Euro per person or best to just provide the facts about the cancellation/ re-route and see what BA comes back with?
I doubt it makes much difference one way or the other. I would keep the submission as short as possible, but I guess if the processing agent and you rapidly align on the same figure then it may make it marginly faster to write the reply email to you.