Why? The minimum downgrade compensation is set out by law, and not too difficult to claim. You're entitled to 75% of the cost of the ticket (both Avios and cash) refunded to you.

BA would like to take the position that the "cost of the ticket" refers to the portion of the ticket paid for your specific flight downgraded rather than both, but if you've paid for the return flight in one booking there's no harm other than a c£30 fee to get BA in a small claims court and getting an independent judge to decide.

OP - sorry to hear of your disappointment, it's a real pain when you've been looking forward to it so much. In your shoes I'd be mentioning the key words EU261 to BA - expressing my disappointment that you've found out that BA did not inform you of your rights as they were supposed to do, and whilst you could claim a refund of the whole ticket, you are happy to get 75% back of the one way trip cost and a space available upgrade on your next flight instead if you can get this settled by 31st January.