EC 261/2004 is payable by the operating carrier. If there is some transaction with the TA, then that is between BA and the TA and is none of your concern.

The calculation is 75% of the base segment fare paid. That ought to be 75% of your J segment stripped of all taxes and fees + 75% of your GUC which is essentially the return of the GUC. The 75% paid in cash (equivalent) unless you negotiate with BA for something else.

I would make a claim for the specific amounts due and 75% of the GUC (let BA simply return it or offer something in lieu). Send that in the form of a Letter Before Action (forms available on the MCOL website). Fighting about this without the Letter is simply time-wasted as one must eventually send the Letter before filing, so might as well use it as the "escalation."

Make your Letter crystal clear and be very clear about what you requested as opposed to what BA offered. Remember that you likely could have requested to be put on the next F service, but would have waited longer. If you accepted the J service in order to avoid a longer delay, that is hardly a downgrade for purposes of the Regulation. If, on the other hand, BA refused to rebook you onto the next available F service, that is a different issue.

You then have the choice of CEDR or MCOL. The former may be preferable because BA has bound itself to CEDR decisions, but you are not. Thus, if you prevail at CEDR, BA will pay you. If BA prevails, you may accept that or proceed to MCOL anyways. All a choice for you.