You could no longer receive child benefit for a child who is now at university or in full time work, so couldn''t claim child maintenance for that child through the CMS. You also couldn''t class that child as dependent if you claim other benefits such as housing benefit.
A child at university can technically apply for financial support from a parent through the courts in their own right however.
If you have a Consent Order to pay maintenance, things very much depend on the wording. For example "until the child ceases full time education" could cover degree and postgraduate study, whilst "until the child ceases full time secondary education" probably only covers up to the end of A levels/equivalent.