From London Marathon website:
Course cut-off times
We are required to keep to a strict schedule for reopening the roads to traffic, therefore you must meet the cut-off times stated below.
The Course Closed Vehicle travels at a seven-hour pace and crosses the Start Line at 11:00, moments after the final participants. If you do not stay ahead of the Course Closed Vehicle, you will be required to move onto the pavement to complete the event. Further information is available below.
Location 7:00 hour pace
(minimum pace needed to avoid completing the event on the pavements) 8:00 hour pace
(minimum pace needed to receive a medal and official finish time)
Cutty Sark
(6.5 miles) 12:50 13:10
Tower Bridge
(12.5 miles) 14:20 14:45
Canary Wharf
(18.5 miles) 16:00 16:40
Tower of London
(22.7 miles) 17:05 18:00
The Finish Line
19:00
19:00
The above from:
https://www.virginmoneylondonmarathon.com/en-gb/final-instructions/course/
To obtain a finisher’s medal, you must complete the event by the official cut-off time, which is currently 19:00 on Sunday 26 April 2020.
Please note that if you are slower than seven-hour pace (which is 16 minutes per mile), you will complete the event on the pavement. Seven-hour pace begins when the last participants cross the Start Line, which is predicted to be at 10:50 in 2020.
We are required to meet a strict schedule to reopen the roads of the capital and get London moving. This means that if you are slower than seven-hour pace, you effectively become a pedestrian and you will be sharing the pavement with others. There will be a huge clean-up operation on the roads around you to enable us to reopen London to traffic.
https://www.virginmoneylondonmarathon.com/en-gb/event-info/participants-disabilities/