Using your reward miles, don't think it is a totally free seat, only to be told, the seat is free but you have to pay the taxes - in the case of London Bkk London, the taxes are almost 50% of the actual ticket price in certain circumstances, with Thai anyway, the only way around this is upgrade to business class by using your miles where the upgrade for taxes is minimum, but nevertheless still has to be paid.
Thoughts on other airlines.