When my cousin was 16, she was bullied really badly by "friends" that turned nasty on her. They made her life hell for months on end until she couldn't take it anymore.
She was home alone, and raided the kitchen cupboards for any type of pills she could find, from paracetamol to ibuprofen and even to her sisters asthma tablets. She took an overdose. Her dad got home just in time to find her on the sofa, unconcious, and had an ambulance rush her to hospital to get her stomach pumped. She survived, but she said to me that she was definitely on the brink of death.
4 years later, she's now finishing a course at the local collage to become a nurse, and has just been accepted by Queen's University Belfast starting this February. She doesn't talk about what happened very much, but she's made it very known that she's now the happiest she's ever been in her life and she's so glad her Dad got her to hospital in time for her to survive, because she said even though at the time it seemed impossible, she has a rich and full life ahead of her, and she realises that now.
Back to your post, OP, don't do it. I have been clinically depressed for the past 2 years, also suffering from spurs of Depersonalization Disorder due to high anxiety levels due to the same thing that happened with my cousin, bullying from a former best friend who turned on me, but I've been able to get on with it and cope, and now I'm starting to get better. I still have my bad days...but everyone does at some point.
In relation to your subjects and results etc, don't worry too much, a lot of universities do an Undergraduate Science Foundation degree which accepts people who've underachieved but want to do a science degree afterwards. The foundation degree will reteach you the basics and, providing you pass at the end of the year, you'll automatically be put into your chosen course, in your case Veterinary Science. My science teacher got 3 E's in his A-levels (Maths, Chemistry & Biology) and was able to do that foundation course and then went on to do a degree in Marine Biology. That's one of your options if worse comes to worse, but you could just get on with your life this year and really work your ass off and get the grades you need. Do it for yourself, not anyone else. And when you get your great grades next August, you can stick two fingers up to all the people who doubted you, and I guarantee, you will feel on top of the world.
Please don't do anything silly, this time next year you will be thinking God, I'm so glad I didn't go through with it, I got my university placement and I'm ready to start my life fresh in September.