It is not impossible to get a top graduate job with poor A levels.
If you have less than 24 UCAS points (300 on the new system i think) then you will have a harder time in getting accpeted to top graduate scheme. That is fact. In such cases, you need to work hard on your competencies, experience and extracurricular activities to make yourself stand out. This is so that recruiters don't automatically bin your application for not meeting the A level requirement (although some will still bin it). Your CV will need to be much better than those who meet the requirements in order of standing a chance of having your application progressed through.
You can still apply to many schemes even if you don't meet the requirements. Explain your situation to HR. I underperformed during my A levels (although if I got to retake like you can now, they would've been all good) but I worked on making my CV stand out once I realised how much a problem by A levels would be when trying to get top jobs. I didn't always meet the requirements but still applied. In the end, I've been offered jobs at IBs (yes, front office aswell surprisngly), consultancies, oil companies etc. It was hard especially when I had no contacts in any of the industries and come from a family in which I was the first to go beyond GCSEs. Enough of the sob story, people need to stop complaining/ dwelling about failure and working on ways to improve their prospects.