I deal with families returning to the UK having worked abroad.
The simple answer is that it is complicated.
The most obvious answer is that you need to be resident in UK for 3 years to be a UK student.
The less obvious answer is what it means to be resident.
If you are on a temporary overseas contract, and have always intended to return to the UK and consider yourselves to be UK residents, then it is possible for your kids to have UK status, even if they come back aged 18.
You need a fair few things to prove this. It is easier if the company you work for is a UK one, they can provide you with a letter stating that you overseas asignment was a temporary one for x number of years (and that can be a long time, eg 10 years)
It is easier if you have a house back in the UK, as that proves you intended to return.
It is similar to the tax status where you have a difference between resident and domiciled, the second being where you are based, the first being where you are living right now. You need to prove that as a family you are domiciled in UK (but they use the word resident for student applications)
It is NOT uniform across all universities. Each one can make their own choice about their decision to allow you to be considered resident or not. The only guranteed way is to return to UK.
I have a query over education, an over seas student who is in school in UK does not accumulate residency if they are here for 3 years, as they are only here for educational purposes.
However with regard to a UK student who returns for sixth form (and for example their parents are still overseas) I am not sure if those years would be counted or not.
It is certainly easier to prove residency if the whole family return, even if that is at the point of university entrance.
It is a bit of a lottery, some universities are very generous and some very tight on the criteria.