Sounds to me as if you have a valid booking and a valid contract - I know of nothing in the rules that redemptions may be cancelled after booking.

I'd be talking now to a consumer protection law helpline to confirm your rlegal position- methinks that the initial issue of the tickets proves the contract and that bmi is acting as an agent for the other airlines. I believe it is down to bmi to deliver the flights as agreed, or you will have a strong case to recover any additional costs in making alternative, equivalent, arrangements.

It's not a case covered by the EU regulations, as there has been no flight cancellation or denial of boarding: the reservation has been deleted and the operating airline is not (yet) at fault.