Good spot. A little more information:
The total number of BA Miles outstanding at March 31, 2006
was 127,638,125,986 and the total number of AirMiles
outstanding was 7,666,093,423. The Company has recorded a
liability for the awards relating to the flown mileage credits of
£15 million and has deferred revenue of £359 million relating to
the sale of miles that are unflown. The estimate of unflown miles
is reviewed and if necessary adjusted each year. In financial
year 2006 this review resulted in the release of £31 million to
other revenue.
The number of frequent flyer RPKs as a percentage of total RPKs
for the years ended March 31, 2006 and 2005 was 2.8 per cent
and 3.2 per cent respectively.
The Company believes that the displacement of revenue
passengers by those travelling on frequent flyer awards is minimal
based on the low percentage of frequent flyer RPKs to total RPKs
and the Company’s ability to manage frequent flyer capacity.
Suggests miles earnt by flying isn't very expensive to them: i.e. the sum total liability for miles earnt but not spent to date is £15m (0.4% of market cap), and presumably the auditors ensure it's a valid estimate.
And selling miles is a nice little earner: £31m of revenue in 2006 from expiring miles worthless. Only 0.4% of revs, but presumably 100% gross margin. (But that doesn't account for them being sold above/below fair value which would be an immediate gain/loss.)