All universities will have some students in their own managed accommodation (let alone those in second year and beyond living off-campus) who won't have been able to go anywhere over the holiday period, for various reasons.
The section referring to who should be allowed to return during the first three weeks also states:
HE providers should also consider allowing the return of students who may need to return earlier for other reasons, for example, students who do not have access to appropriate alternative accommodation, facilities, studying space, or that need to return for health reasons. In these circumstances,HE providers can make plans to welcome these students back on campus from their original start date but their courses should not resume face-to-face teaching, unless they study one of the practical courses defined above.
As part of these plans, providers can consider what provision they can make available to such students, along with those students who remain on site over the winter break, including international students and those who commute in on a daily basis. This includes access to resources, study spaces, campus catering, specific support required by international students, and appropriate pastoral and study support in COVID-secure environments. These students should be tested as soon as they start accessing university facilities.
So basically, more or less anyone can actually be 'allowed' to go back at the normal beginning of term, even if there is no in person teaching for their particular course for the first month or so. As far as I am aware, the only legally enforceable part is the requirement to self isolate having had a positive test.