Subject to the agreement of the individual, and except for degree certificates, a written notification of intent to transition is sufficient for the gender and name on staff and student records to be changed. This is necessary to enable the person to go about their daily life as a staff member or student without their sex assigned at birth being known.
This is from Bristol Uni. I guess, regardless of any other policy, if they allow students to register or update their sex based only on a signed declaration, the accommodation office have no way of reliably allocating same sex accommodation.
They do have a single sex accommodation policy but that is pointless if they allow students to change their sex on their records.
At least if it is not a deliberate policy (I think we heard from someone who works at another uni with a same gender policy?), then it might be easier for someone to complain and get moved.
Presumably, if they did gather the relevant information, they could ensure that transwomen get to share with each other :-)