Ok so if you book through a travel agent you dont get correction or cooling off are the big ones.

Then after that changes like date/time/rebooking after cancelled flights schedule change if pre travel/name correction/upgrades would all go through the travel agent.

If you book a published fare changes would be possible through BA mid travel (once first flight is taken).

If you book a deal/bulk ticket or any other 'special' fare set up by the travel agent even mid travel you would contact travel agent for changes of date or time or upgrades.