Honestly, I think your post has kinda given you the answer already. A top UCAT score will definitely help, but as GSCE is also weighed quite heavily, I would say your chance of getting an interview is probably around 40%, that is if you scored top marks in UCAT (idk if your school is classed as above/below average in your post, if it's below then your chance of getting an interview will be higher). But even getting an interview doesn't mean you will secure an offer, so the actual chance of getting in is even slimmer. If you really want to study dentistry, maybe you shouldn't limit yourself to London, whatever your personal reason is. But if that reason is really that significant, then I would advice you to choose other courses that is related to dentistry and more available in London. Please don't just take my opinion as I don't know how accurate my view is, ask your teachers/parents/ friends who got experience. Hope this helps!