Clearing is not "call them up and beg on results day". It's a specific process whereby universities will advertise they have places available in clearing for specific courses. Historically UCL has not participated in clearing, it did participate in clearing plus last year (where universities will invite eligible applicants in clearing to be considered for an offer, and is not open to others). It's unlikely they will be in clearing this year, and much much less likely even if they are in clearing or clearing plus that medicine will be in clearing.
Also if you are holding an offer you can't go through clearing, and rejecting a medicine offer to try and get one from another medical school is frankly stupid, since where you do your medical degree has no direct impact on your ability to get a job in medicine. The GMC considers all medical schools equal and the NHS is the only provider of graduate medical training posts, and takes the GMC stance and to ensure there is no bias, blinds recruiters from your medical school.
The hard part is getting an offer, stop faffing about thinking about nonsense like "dream universities" and such and focus on meeting the offer you have and getting on with becoming a doctor. The medical degree is at most 6 years of a potentially 50+ year career. The important thing is to get it at all, not where you get it.