Sorry for length. Please feel free to ignore...
You need two plan Bs and at least two plan Cs.
Plan B(i) - What do I do if I have no medicine offers but get the required grades? This is almost a no-brainer: you do UCAT again and reapply more strategically. If that seems like too awful a prospect, change direction because you ain't got the guts for medicine (degree or career).
Plan B(ii) - What do I do if I don't get the grades, regardless of whether I have any offers? There's a very large number of students in this position this year, but there's always a fair number. Anyone who assumes this can't possibly happen to them is both naïve and arrogant. This situation is more difficult because resitting A-Levels can be complicated, quite a lot of medical schools won't accept applications if you're resitting A-Levels, and those who will might be the ones you have no chance of getting an interview at because of UCAT requirements. If resitting isn't a realistic prospect - e.g. because you're just too far away from the required grades - you might have to consider doing another degree, which might mean taking up a UCAS 5th-choice or an alternative course offered by your original firm-choice institution.
If you do another degree, you must acknowledge the following two points.
(a) The chance of being able to transfer to medicine from another course is, for all practical purposes, zero. Most medical schools won't accept transfers from anywhere, not even from another medicine course. Some say they will accept transfers from other courses in their university, but the numbers they allow to do this will be tiny; they are also unpredictable (might be 5 one year and 0 the next). The exception is Clinical Sciences at Bradford, which has formal arrangements with local medical schools for a transfer process, but far more students apply for these transfers than the number of places available.
(b) Applying for medicine as a graduate is at least as competitive as applying as a school-leaver, and university careers services will provide much less support than your sixth form did. You also have to submit the application yourself, get someone to agree to be a referee and then actually make them write the reference. I've had quite a few applications from graduates that we've rejected because they didn't get a reference submitted on time. Some academics are unreliable and will often refuse to do stuff if they don't think it's important, so getting them (i.e. us) to write references that they don't feel are useful for students who they might not know very well isn't an easy task. (We had one graduate applicant whose reference stated that the referee had no way of knowing whether the student was a latter-day Mother Teresa or an axe-murderer.) Anyway, the point is that there is no guarantee your graduate application for medicine will succeed, so your first degree must be of value to you in its own right. Don't choose a degree just because you feel it will sound good to a medicine admissions tutor: make sure its something you can use if medicine doesn't come off. Some medical schools will only accept certain first degrees. The most obvious is Cardiff, which has designated feeder degrees for its graduate-entry programme, but some others will say the first degree has to be science or medicine-related science. Others will accept any first degree as long as certain A-Level and/or aptitude test requirements are met. You need to do this research before you start the first degree, not at the start of your final year.
Plan C - If medicine just isn't going to be an option, what are my alternatives? the answer to this will vary greatly among individuals. Some will want another professional role, so if medicine isn't possible they will do law, accountancy, etc. Others will want another patient-facing healthcare role and will look at pharmacy, physiotherapy, radiography, optometry, or any of the other 350 careers in healthcare. Bear in mind that some health professions require postgraduate qualifications and might be open to people with a range of bachelors' degrees, but some of the postgraduate courses only accept people whose first degree is also a health professions one (most commonly nursing). Other students will want to pursue something related to medical science. It's important to know something about your preferred alternative before you start out on the medicine trek, otherwise you will panic if something happens that makes medicine an unrealistic option. Remember: only about one third of people who apply for medicine get a place.