You are correct in that you should get 75% of everything back for a downgrade, with the exception of actual taxes (eg APD) and fees (eg Airport departure fees). The only slight point where you're wrong is in calling it "compensation" - it's actually a reimbursement. That doesn't change anything in practice, but best not to give BA the slightest excuse for denying you the cash and avios.

If it comes to CEDR/MCOL then remember to covert the avios you're owed to cash at the rate BA are selling avios for...