You get to keep your AS grades.
However if you want to retake your A levels, you'll probably have to pay of you are over 19 when you start again.
College will consider taking you back, they take many older students, myself included. Many people mess up first time and have to retake.
Universities will take you if you can show you have changed over 2 years and do well in your A Levels, so yeah. Just focus on getting your A Levels done properly first.
Another option is Access to Higher Education. You can look in to that but it's mainly for older students who are returning to education after a while.