@Raffles - I suspect (?) that these sort of deals (eg the £1500 J class you mention) are only offered to high-producing, trusted agents.

There was a time when many airlines would impose MSP /‘Minimum Sell Price’ restrictions within their agency contracts, primarily as a means of protecting their less influential, less powerful agents ; their own internal distribution channels ; and their public image. This did not however prevent an airline offering some extremely attractive nett fares - with the agent then free to determine its own mark-up - subject to any MSP, of course.

But perhaps with the ever-increasing power & value of the OTA’s, such restrictions have become less common. Feel free to put me right - this is far more your area of expertise/knowledge than mine !