But even if you are you are at the front of the queue.
For LMs you can get onto the platform and nobody cares. Commuters generally know the platforms and go down early anyway, they are very consistent. (They also know which inbound train forms it for a quick check on the screens, or use RTT though be careful where the platform is not bold).
For VTs, if you do get down and they're still cleaning it don't board until it appears on the screen, you'll just get in the way of the cleaners and the reservations won't be placed. You'll still be ahead of the scrum.