Okay I see - I thought you were talking about the Airport sending the line rate upstream.
The only visibility the Airport or any other end user device can have of line rate is it being sent as part of RADIUS attributes or DHCP options. The Openreach equipment inserts line rate information as either a DHCP option or PPPoE Active Discovery Initiation.
The Openreach kit also removes tags sent from CP to equipment and removes anything in DHCP option 82. It doesn't really care what the CPE or CPE have to say
PPP timeout being higher than the time it takes to complete a retrain would stuff things, though, always.