If you read carefully the UCL site you see that despite the deadline for application is May the actual allocation of the rooms available is done at the end of August (after A levels publications) once they know exactly who has been able to satisfy the condition. If you fail to satisfy the condition you will not be offered accommodation and you will seek to secure lodging in your insurance choice. That accommodation will most likely be offered to someone who has put UCL as their insurance and, having failed their first choice, are now seeking to attend UCL. I have recently spoken to an administrator at UCL and he explained that this is exactly how things happen. You may not make your offer, others may not make their offer to say Oxbridge and attend UCL instead. So it's a musical chair game till everyone is sorted.