i mean this in the most respectful manner possible, but a-levels are not worth killing yourself over. they are incredibly stressful, i won't deny that, but if that alone is what's pushing you to think these thoughts, then i think you might just be a little burnt out from all the studying you've just finished. i thought i bombed mine, but i ended up doing alright -- not as well as i could've done, but well enough -- and still got into my first choice. my sixth form friends all thought the same and all managed to get into their first choice. if that's what you're worried about, i hope that's put you at ease.
you sound mostly afraid about an uncertain future, but even if you have fucked up these exams, there are so many choices for you. you already have gcses, which means you have a wider scope than a lot of people do. you could even resit your a-levels if you wanted, or reapply to uni after a gap year. you have a lot going for you, especially since you got your 'dream offer' (which is a great achievement in itself) so please, respectfully, reconsider this irreversible decision and don't give up just yet. you're not eternally doomed