In the UK HMRC does not consider airmiles earned by an individual during work travel to be a benefit in kind or taxable in any way, unless they are only awarded to a person because of their employment (e.g. if an airline had an XYZ Company scheme that awarded XYZ Airmiles, not open to the general public).

http://www.hmrc.gov.uk/manuals/eimanual/EIM21618.htm

Morally, I have no compunction whatsoever collecting Avios (or any other frequent flyer miles or points) when flying on business travel.