As in science GCSEs and B in Maths will hold your application back unless you have a really good reason for it (ie mitigating circumstances). All through school we were told "don't apply if you don't have 6 or more A*s at GCSE," and indeed I've never met anybody who got less than 7 who has successfully got in for medicine. These places give offers to an extremely low percentage of their applicants (
http://university.which.co.uk/search/course?c[q]=medicine), and the easiest way for them to determine who they're going to interview is with the GCSE/AS tick box.
Absolutely some people get in with less, but if you rock up with As and Bs at science/maths GCSE then there will be big question marks. You can't offset this with work experience, because all realistic candidates offer this. If you get something like 800 on your UKCAT this might offset it, but still, to not get A* grades in these subjects, which compared to what you need to do in a medical degree is trivial, shows either that you're extremely unlucky, that you didn't prepare well enough, or that you're not clever enough. They don't normally take the chance that you were just extremely unlucky. They might if you offered a portfolio of A*s with an obviously anomalous slip-up, but not if your baseline is lower.
I don't want to sound harsh or anything, but I've seen really quite a few people with majority A*s at GCSE, great work experience and extra-curriculars, strong UKCAT, all As at AS etc. fail to receive any offers for medicine. But you don't have to do medicine to succeed in life.