Originally Posted by Often1

It would be one thing if BA did what most carriers do in this situation, that is to tentatively protect on the next available service pending the passenger's acceptance, but not treating that as a rebooking until the passenger has acted (which could include simply clicking a box online and checking in).

As most passengers do want the first available service and would likely accept the downgrade, it makes sense to hold the space before the passenger's options are foreclosed. But, that should not be treated as the passenger's acceptance. A passenger might choose one of the other EC 261/2004 options (also covered in BA's COC in plain English). By way of example, if one cannot fly CE and one must also wait, why not simply purchase a cheap ticket on an LCC if one is available? That, of course, requires selecting the refund option from BA.

As OP's profile shows him living in the UK, he should simply commence the MCOL process for 75% of the segment price. The form for a letter before action is on the MCOL website and that may prompt BA to action before the filing itself is made.

I would focus on the specific which is simply that BA cancelled, made the change and that the change was not voluntary in any sense under either EC 261/2004 or BA's own contract.