What happens to them?
As soon as they have been caught, they then are interviewed by UK Border officials.
Either before or after that they are given medical treatment if required and clean dry clothes if needed.
It is also possible that they will be fed with some food, this will probably be fish and chips or something similar from the most local place able to provide this food.
They will then be taken to immigration centres anywhere in the UK where they will be able to eat and drink at the expense of the UK until such time as the system makes a decision on their claims, which could take years.
If released into the community meanwhile they will be able to get a small amount of money each week for living expenses and their lodgings will be paid for.
This may not happen straight away.
In the meantime they will be able to receive treatment for any medical conditions.
If returned to their place of origin they will be flown there with several escorts depending on how the UK government has assessed them.
This can only happen if they accept this offer or if their asylum claim is refused, which will take several years.