BA are fully aware of the fact that redemption bookings are still quoting the old GB tax cost. This was overlooked up until about a month ago. Its u to the agent to change the GB tax amount in the TST to the correct level. In the last month, UK distribution have been passing these bookings back to agents to call out pax to collect the additional money. If the pax refuses to pay then the agent has to account for it. Your gain is somebody else's loss.