Apologies for the Wall of text.
I'm in a similar position to you. I will be 23 when I graduate from Nottingham in Management Studies and plan to apply for their GEM course starting 2012.
This link shows every GEM course in the UK and also has other useful things in there:
http://www.medschoolsonline.co.uk/index.php?pageid=11Having a 1st already definitely puts you in a great position because you can apply to 5 of the 6 Unis that don't ask for: GCSE or A level grades, and dont care what subject your first degree was in as long as you got a 2:1 or higher (or just 2:2 in the case of Notts and St Georges). The Unis I'm referring to from the link are: KCL, Swansea, Keele, Newcastle (2:1+ unis).
For Nottingham, Swansea, St Georges and Keele you will need to sit the GAMSAT test which is basically an aptitude test that tests you on your: verbal reasoning, essay writing skills, and science knowledge. Coming from a business subject you'll probably find the essay writing part easy, and verbal reasoning can be practiced just by reading good newspapers and books regularly. Science on the other hand needs to be learnt, I'd recommend teaching yourself A level: physics, chemisty and Biology + get the Organic Chemistry for Dummies book. This will give you all the knowledge you'll need to score heavily. Also buy the Acer GAMSAT practice questions to familarise yourself with what the test is like.
For the other 2 Unis you need to take the UKCAT which from what I'm told is a walk in the park compared to the GAMSAT as it doesn't require any revision, just practice answering their style of questions. However getting places into these Unis is more competitive because both Unis that offer the course that you meet the reqs for take less than 25 people each year.
I would advise focusing on the GAMSAT because even though the exams tougher you're increasing your chances of getting a place. For example; Notts offer 91 places, St Georges 98, Swansea 70.
I read an inspiring story from an online article last year; that a single mother of 1 studied for the GAMSAT for around an avg of 5 hours a day for 3 months, and scored over 80 percentile in the exam (with no prior Science knowledge before her revision since GCSE's) and now is on her 2nd or 3rd year in Medicine.
The GAMSAT registration for their 17th September 2010 sitting opens sometime in June which is definitely worth a punt. From June-Sept 17th if you put in something like 4hrs a day revision or abouts will definitely have a great chance of getting a good grade. Also another great thing about the GAMSAT is the marks can be used for upto 2 years, so if you don't make it through the interviews the first time you won't need to sit the GAMSAT again, unless you don't make through the interviews the 2nd time as well.
This here is a great site where lots of GAMSAT students exchange tips etc:
http://pagingdr.proboards.com/index.cgi?board=gamsatStudying aside make some time for gaining some work experience as well, 8-16 hours a week for a period of at least 3 months at a Care home will be great experience. I have been told from an inside source that they value care work over gp shadowing or hosptial work due to its direct caring nature. So prioritise getting Care home work over the others...
Take care,
James