The nights are drawing in and it is getting colder, but hedgehogs still have plenty of time to put on enough weight to get them through hibernation. The time that hedgehogs go into hibernation can vary not only from year to year but also from region to region. It can be from late November to early December before hedgehogs think about hibernation but if the weather is mild (like last year) then hibernation can occur much later. This poster from a collaboration of Wildlife Rescues (October 2020) has excellent information.
Rescues are currently being inundated with hedgehogs needlessly brought in by members of the public being concerned that they are too small. It is very important to remember that there is still plenty of time for the hedgehogs to feed up so continue to leave out suitable food such as Brambles Crunchy &/or Meaty Hedgehog Food, and water. They will start to think about hibernation when there is a spell of overnight temperatures below 5°C, there is frost, and the ground is hard. At this time hedgehogs less than 600g may need taking to a Rescue but contact your local Rescue for advice first as they will know the local conditions.
Hedgehogs that weigh around 450-600g will usually have built up enough fat reserves to survive hibernation but even then, there is no guarantee of survival. If you are concerned about the weight of a hedgehog in your garden this chart from the RSPCA gives a good indication of size. It really is not a good idea to get hold of a hog in your garden and weigh it as this causes a huge amount of stress to the hedgehog.
Even in normal winters (whatever that is!) if hedgehogs do hibernate, they will have periods where they awaken to look for food, water or maybe a new nest before they go back into hibernation again. So, if you know that you have hedgehogs in the area it is a good idea to leave out dry kibble biscuits that can be left out for a couple of nights if kept dry.