I don't know how international students apply, so I will briefly explain UCAS (if you use UCAS, please feel free to skip this bit)
UK students apply to UK universities through UCAS (University & Colleges Admissions Service) and you apply to 5 universities (only a maximum of 4 can be for medicine). The universities will then make offers. Since not all the offers made by the universities are accepted, they make more offers than they have places. If you accept an offer from a university, (and in the case of a conditional offer meet the conditions) then the university MUST take you on that specified course, whether or not they have room. An offer is a legal contract.
When you are made your offers, if you have all your qualifications, an offer will normally be unconditional. If you do not, then usually your offer will be conditional on what grades you achieve. You then accept your offers. If you accept an unconditional offer, that's it. If you accept an conditional offer, you then have the option to set another offer as an "insurance", in case you do not meet the conditions of your original ("Firm") offer.
Universities do not want to be your insurance. If you select them as a firm offer, generally most people meet the conditions, and they can make arrangements for your arrival. If you reject them, then they obviously won't. If you set them as an insurance, it makes life difficult for them logistically.
Therefore, universities hold "UCAS days" in which they invite applicants to see their university department in more depth (read: advertise to you) in the hope, that if they made you an offer, then you'll put them as a firm.
Many universities (even if they don't say it) are reluctant to give an offer without advertising to the student. If you get an initiation to a UCAS day, GO! The first thing said to me at my UCAS day in Southampton was "Ok, since you've all come, we're making you all our standard offer". For some universities an invitation to a UCAS day means that your application has made it past stage 1, and the university is taking your application seriously