Edit - Looking at the route there's a lot more water than this. However I've trained to this now so hopefully it'll be okay!

For my reference, as my runs get longer. These are the drinks stations in the London Marathon (for 2020 anyway, I can't see them changing that much in 2021). The plan is to have breaks at the same point with the same drink etc as part of my training.

 MileProvided 
 1 
 2 
 3 250 ml Water 
 4 250 ml Water 
 5 
 6 250 ml Water 
 7 Lucozade Sport
 8 250 ml Water 
 9 
 10 250 ml Water 
 11 Lucozade Sport (Compostible Cups)
 12 250 ml Water 
 13 
 14 250 ml Water, Lucozade Sport Carbo Gel
 15 Lucozade Sport
 16 250 ml Water 
 17 
 18 250 ml Water, Lucozade Sport Carbo Gel
 19 Lucozade Sport (Compostible Cups)
 20 250 ml Water 
 21 
 22 250 ml Water, Lucozade Sport Carbo Gel
 2325ml Lucozade Sport Pod
 24 250 ml Water 
 25 250 ml Water 
 26 

The weirdest thing has to be the Lucozade Sport Pod on Mile 23. They've put Lucozade Sport in seaweed pods, and you just chuck one in your mouth and bite!

London Marathon Runners Were Handed Seaweed Pouches Instead Of ...