I do not know if they look at attendance, but your grades are good regardless!
To be honest, if your A-Levels are good enough, than your GCSEs will only have a little to no bearing on if they make you an offer. My advice is to work as hard as possible on your AS-Levels and find the relevent work experiences as soon as possible. Make sure you can get your personal statement as good as it can be- and this means starting now.
Don't let the horrro stories put you off. If you really want to do it then you have to just go for it.
Good luck. If you work hard enough, you can definitely do it!