I would say that it would be immoral and against the terms of the BAEC to actually sell the Avios flights for money or anything else under a contractual agreement.

However it could be fair to come to a gentlemen's agreement that for every, say, GBP 5k you save the company through using Avios for high priced flexible tickets they will spend GBP 3k (since inflexible personal flights are cheaper than flexible tickets) on flights for 'market research' where you would do an hour of market research at the other end of the itinerary and spend the rest of the time on holiday.

This would have the advantage for the company of getting cheaper travel and the advantage for you of being able to travel on flights without Avios availability.

Is it legal? I do not know.

Is any tax due? Probably not, since the flights are for business reasons.

Is it fair? I would say yes!