Was originally scheduled to take BA460 LHR-MAD on June 19, but due to delay in connecting flight was moved to BA462. However, currently unable to select seats or checkin and BA says that is due to June 19 BA462 being under airport control so I have to wait until reaching LHR to get seats and checkin. That doesn't seem right since I should be able to do that at the departure airport during my US-LHR flight today, June 18th. Am I missing something?

Usually when there is IRROPs and rescheduled connection BA books me onto another flights, assigns seats, allows checkin so not sure if there is something more I should be concerned about.