Hello there,
Firstly good luck with your GCSE results!
1) As I'm starting med school in sep I can't comment on the stress level, yet I have just finished another degree and that was hard graft in itself. But, it's totally worth it, and the time you spend studying at uni is so much more than sitting in a library!
2) The entry requirements for medicine are fairly high, but whats more important is the EXPERIENCE (key word

) that you gain working in the healthcare environment and UNDERSTANDING what being a dr. actually is. But as a guide, and to keep you motivated during those laborious A levels, most med schools look for ABB-AAB-AAA in your A levels.
This is where it gets slightly more complex, because each med school wants different things from you. As a rule now though, you will prob. need to take an admissions test called the UKCAT (uk clinical aptitude test), submit a statement about your reasons for wanting to do medicine, and an interview.
3) GEP is an option, although you will need to do a degree pre-medicine to qualify. It's a good choice if you miss out on an admission straight after a levels, or need some time to get more experience and make sure you want to be a dr.
GEP are fairly difficult to get onto, but again nothing is impossible with the right candidate showing understanding of the career. GEP is taught slightly differently in most places, with more clinical based and less basic sciences.
A distinct advantage of GEP is the NHS funding from year 2 of the course, opposed to year 5 in the normal course.
4) Pre-med courses are for people without the necessary science background (eg no A level bio/chem), but with other good A levels/qualification (eg Art, music, english, psych etc). They basically are like a yr0, before being accepted onto yr1 of a medical degree.
Most important...make sure you pick the A levels you know you will enjoy and do well in...not just for a medical application. And get as much experience as you can (it really is the most important part of your application-so start early!!)

I'm around to answer questions if you have any,
have a goooood day

xx