but one of the interesting things is that if you fly on behalf of an employer (but not hired as a pilot as such) then the employer can contribute up to ALL of the direct costs associated with the flight.
Better than that... the employer can provide you with the whole aircraft, servicing, fuel, landing fees, the lot.
If it wasn't so, an aircraft owner who flies (to take the simple case) 100% on the business of his company, would have to foot some of the costs personally, which would be daft.
However, the other aspect of this, in the UK, is the Benefit in Kind system which the Revenue loves to invoke to screw aircraft owners for 5-digit settlements. Been there, done that... You need to get expert accounting advice here.