score:6
The EC 261 calculation is based origin-to-destination so it is with respect to your final arrival in ABZ (unless you originally had a stopover in London). So one payment applies for the delay, but it is based at the higher rate.
Also according to EC261, either 30/50/75% of the ticket price should be refunded when you are downgraded, but it's not clear if this applies to the whole ticket, or just the affected segment.
Although the wording seems to imply the whole ticket should be refunded at 75 % (and I know at least one person who tried to claim 75 % of his $8,000 round the world first class ticket when a 30 minute flight was delayed), recent European court cases have clarified that the downgrade compensation should be pro-rated based on the the downgraded segment length. This is also the opinion of the UK Civil Aviation Authority. In this case I think you would be entitled to 30 % of (total ticket price * LON-ABZ distance / total distance), in addition to the cancellation. Based on the distances on gcmap.com, I work out the downgrade compensation as 0.04*0.30 = 1.2 % of your total ticket price.
In total ticket price, I would include both the Avios and the cash (including the upgraded cash you paid), but I would not expect to receive the Avios back.
Claim for both and keep your explanation succinct and clear.
In the event that this compensation does not cover your actual expenses that you incurred because of BA, I suggest that you send in any receipts you have with a demand for further compensation to cover these costs. BA will pay out anything "reasonable" even if it slightly exceeds what they said they would pay in the literature. [However for some bizarre reason they do not pay dry cleaning bills so you are better off buying new clothes than cleaning the old ones.] But, if you have travel insurance to cover these costs, then claim for these from your insurer instead.
I also suggest you take a look at the expert guide on EC 261/2004 on FlyerTalk.