This post made log in almost 5 years later to stop you making a mistake I made.
Listen up.
I rushed into University, I had decent grades but I decided that my reaction to 4 medical school rejections was to rush off an do a course I barely knew about. The result? I'm a 3rd year student on a course which I was never truly interested in. Now that I've been at Uni for some time I've realised something. Years don't matter. Seriously. There are plenty of Medics who took a gap year (or two!) before they applied, people who've been rejected multiple times before they get in. Your career will be 35+ years long in Medicine, waiting a year or two to improve your grades won't hurt (in the long run).
Another reality is that if you do not get into the GPEP/GEM (4 year programme) you will have to self-fund the 5 year programme. Think of it this way, 1 year out + 6 years (including intercalating) or 3 years (on a degree you may not like) + 4 years (on an insanely difficult programme to get into which you must part fund). It's the same. Plus with a year out you enter Uni more mature and with access to a student loan.
To conclude, you're stressing and thinking way too far ahead. I can totally understand the way you're coming from, but no-one will judge you for re-sitting A-levels (anyone who does you can ignore)*. I've seen plenty of people do so and get into more competitive courses - ultimately helping their career. Regarding specialties, a lot of people I know have ended up throughout their time at Medical school. I would be hesistant to make such solid plans (especially childbirth!) at 18. Why did I post this all? Because years ago just like you I thought I'd go do a degree and get into GPEP easily*2. The reality? I didn't get into GPEP, but fortunately have a place on a 5 year course starting this year. How I wish I'd not be a wuss, ignored (what I thought) other people would think about me and just taken a year out.
*Apart from Med school. Do care about what they think about re-sits when applying...
*2 I'm not saying you will never get into GPEP/GEM, but the odds are low and its a risky venture. Everyone told me so when I was in Year 13, but I ignored them. Check the competition ratios if you don't believe me. You can totally avoid this insane competition, by just taking a year out and it won't take you any longer.