Oh and despite what is mentioned above... the Army is currently at full manning... which means that they recruit less and can be more picky. Think long and hard about what job you want to do and which regiment you want to join. My regiment won't be recruiting again until Sep 2010 as we currently have 132 extra blokes!
Also if you have the grades seriously consider commissioning as an Officer rather than going in as a Private.
You're best bet is to talk to an Army Careers advisor.
Minimum Physical standards are:
Males
under 30: 1.5miles in 10mins 30secs (Ideally you want between 8:45-9:30 any slower and you'll be struggling during training. Sub 9mins is usually regarded as being 'fit' sub 8:15 is 'very fit' and sub 7:45 is maybe 1 in every 200 soldiers.
Press ups: Total in 2mins - 44 Minimum. Aim to be able to do 60. 70-80 'fit', 80-95 'very fit' 95+ again rare but does happen.
Sit ups: Total in 2mins - 60 Minimum. Aim to be able to do at least 75... I've seen guys score over 120! however they had really bad technique and any strict PTI will stand over you and just repeat ONE ONE ONE ONE ONE until you correct it then he might change to TWO "keep you fecking elbows in" TWO - THREE etc
The fitter you can get the better. The best fitness program for potential soldiers British army wise is probably on the Royal Marines Website, failing that there is a book called Fighting Fit by Adrian Weales that is just training programs and guidance on military techniques etc.
Good luck! 