It's seems clear to me.. (cut and paste from UCL website)
Single Students
All single first-year undergraduate students entering UCL for the first time and requiring accommodation are guaranteed a place provided that:
-they apply for accommodation no later than 31 May of the year in which they intend to enter UCL, and;
-they have received and firmly accepted an offer of a place on a degree programme at UCL, conditional or unconditional, by 31 May of the year of entry, and;
-they have not previously lived in or studied in the London area (i.e. within the M25) as a student at a Higher Education Institution
Students who meet the above criteria, holding a conditional offer, will receive an accommodation offer after their offer status changes to unconditional.
Insurance students
If you hold UCL as an insurance offer, you must be prepared to plan to find accommodation for yourself using University of London Housing Services.
Late applicants
If you are a new undergraduate entrant and are applying late, you must be prepared to plan to find accommodation for yourself using University of London Housing Services.
Study abroad and Exchange students
Full-time undergraduates attending UCL on Junior Year Abroad (JYA), Erasmus or Exchange programmes for a full session are considered equivalent to first-year undergraduate students for the purposes of allocating accommodation. Students intending to attend UCL for less than an entire session cannot be guaranteed a place in residence due to the limited number of spaces available.
Accommodation for couples (married or in a civil partnership) and those with children
UCL accommodation for international students and partners who are married or in a civil partnership is available at Bernard Johnson House, Neil Sharp House and the University of London’s Lillian Penson Hall and International Hall. There is limited UCL accommodation available for students accompanied by children. Students with children are advised not to bring them to London until firm arrangements for accommodation are in place. Applicants requiring more information are advised to contact UCL Student Accommodation.