I just edited - I had another idea - try not to go to Tallinn at all and turn back at Helsinki

I don't think BA has that rule - it's the American carriers that are a lot hotter on this.

I think just be honest and say you are doing it for the points / fun . It'll depend on the agent as to how successful this will be - but considering it was a pretty significant delay they should be quite helpful. You won't get anywhere on the phone with this - has to be airport staffers