Pre-booked seat allocation released at T-13

Hello,
As travel agents, we have now been told what most of you already knew:
Changes to Pre assigned seat process prior to departure ex London Heathrow only
Effective immediately, pre assigned seats in the Economy cabin will be released back into the seat map 13 hours prior to departure on all London Heathrow departing flights.
This means that passengers with pre assigned seats, who have not checked in 13 hours prior to departure will lose their pre assigned seat.
When pre assigning seats for passengers at the time of booking, it is essential that all passengers are advised that their seats are not guaranteed and that they should check in online 24 hours prior to departure to secure their seat numbers. Please note seats remain subject to any operational aircraft changes.
In the event that a passenger checks in online less than 13 hours prior to departure, and finds that their seats have been released, please advise them to select suitable seats that are available at the Online Check In stage.
Passengers that have been unable to use Online Check In will have seats allocated when they arrive at the airport.
Exceptions
The following passenger types will not have their seats released:
Upper Class passengers
Premium Economy
Passengers on the US watch list (and are prevented from checking in online).
Group bookings of 10 or more passengers that has been made directly with Virgin Atlantic’s Groups Department
Passengers with the following SSRs in their booking:
UMNR
WCHR/WCHS/WCHC
BSCT
As travel agents, we have now been told what most of you already knew:
Changes to Pre assigned seat process prior to departure ex London Heathrow only
Effective immediately, pre assigned seats in the Economy cabin will be released back into the seat map 13 hours prior to departure on all London Heathrow departing flights.
This means that passengers with pre assigned seats, who have not checked in 13 hours prior to departure will lose their pre assigned seat.
When pre assigning seats for passengers at the time of booking, it is essential that all passengers are advised that their seats are not guaranteed and that they should check in online 24 hours prior to departure to secure their seat numbers. Please note seats remain subject to any operational aircraft changes.
In the event that a passenger checks in online less than 13 hours prior to departure, and finds that their seats have been released, please advise them to select suitable seats that are available at the Online Check In stage.
Passengers that have been unable to use Online Check In will have seats allocated when they arrive at the airport.
Exceptions
The following passenger types will not have their seats released:
Upper Class passengers
Premium Economy
Passengers on the US watch list (and are prevented from checking in online).
Group bookings of 10 or more passengers that has been made directly with Virgin Atlantic’s Groups Department
Passengers with the following SSRs in their booking:
UMNR
WCHR/WCHS/WCHC
BSCT