where does BA make the claim of 800,000 seats?
that would be 2191 avios seats per day. BA has ~275 planes in the fleet. say 250 fly every day and do an out and back (so 500 flights - so that's 4 avios seats per flight per day. which is actually lower than the 6 avios seats BA promises to release (4 in Y and 2 in Club). That 500 flights is also an under estimate as SH would do more than 2 a day.
That does not take into account any extra seats BA releases.
So I'd say your 1.8% is too low a figure
I suggest 500 flights * 6 avios * 365 = 1,095,000 avios seats per year. Based on 45m passengers = 2.4% plus any extra they release. Which given I've not allwed for extra SH is still an under estimate