Well you'll need chemistry AS and A2. Some universities require an "A" grade in chemistry. Things may change with the introduction of A* grading at A Levels.
Biology IS mandatory at numerous medical schools (i.e. certainly not a myth, e.g. Nottingham -
http://www.nottingham.ac.uk/mhs/undergraduate-courses/medicine-undergrad/index.php). However I would highly recommend that you do biology even if it isn't a requirement. Medical schools look highly on biology A2 students.
http://www.thestudentroom.co.uk/wiki/Where_to_Study_Medicine - This list contains all of the British medical schools including their grade requirements. Some may be out of date as I'm not sure when the last time the list was updated. However, it will give you a general idea. Unfortunately, medical schools are getting much pickier due to the increasing number of applicants so its unlikely that, assuming there is out of date information, the grade requirements have decreased...e.g. UCL just went from AAB to AAA this year (inc. A in chemistry).
Just to let you know, many medical schools only consider further maths and maths as one A level, so depending on your reasons for taking it, it may not be necessary.
Also, many medical schools require some form of examination to be considered. They all use either the UKCAT or BMAT tests, so don't worry about taking a different one for each Uni. The maximum you will take is 2 if you choose one school which wants the UKCAT and one which wants the BMAT.
A great book to help you is "The Insiders' Guide to UK Medical Schools". It helped me a lot in the application process.
Whilst work experience is very useful, it isn't mandatory. The application staff at the medical schools are well aware that many students have great difficulty and bad luck with trying to get placements. However, volunteer work is a must! I can't stress that clearly enough. And I don't just mean helping out at pre-schools and summer camps. You need to show them that you're willing to get your hands dirty in the not-so-glamorous part of medicine. Cleaning up vomit at rest-homes and aiding the severely disabled go to the toilet is something that no one wants to do, but unpleasant situations similar to these will make up a large part of your life as a doctor. You need to be able to show that you understand what you're getting yourself in to.
Before you finish school, make sure you have some extra-curricular to your name. Med schools want people with personalities, not robots who spend every hour of their waking lives in a text book. That doesn't make a good doctor. Positions of responsibility are also looked highly upon: prefects, team-captains etc.
There are some great annual courses at Nottingham University which introduce you to life as a medical student: Medsim and Medlink. A very high proportion of prospective UK medical students will attend at least one of these. When I went, I found that Medlink was more of a basic introduction into what medicine is all about, including lectures on all of the different branches of medicine. Medsim was more hands-on. We learned how to use various medical instruments and had some sessions where we attempted to diagnose real patients. We also learned some basic medical theory in sample lectures.
http://www.medlink-uk.com/
I don't want to worry you, but getting into Medical School is tricky. Don't get me wrong, if you are a good candidate you will get in eventually. The problem is the HUGE number of applicants each year. Any very many of them will be just as good as you are. A lot of the process is, unfortunately, luck. Loads of great candidates apply every year and don't even get an interview. But even in this event, you could take a gap year and reapply, or take a different degree and then move onto medicine afterwards. No matter which route you take, if you are persistent, you will get in eventually.
Good luck mate. If you need help just ask. I'm also a prospective medical student. I'm taking a Gap Year and will hopefully be going to med school next year.