While it’s not ideal to reverse onto a busy road, there is no law against it. Firstly, you would have to determine what ‘busy’ means, which is difficult because many ‘busy’ roads are only busy at certain times of the day, e.g. during rush hour.
The law states that you must not reverse your vehicle if you cannot do so safely and that you must not reverse on a motorway.
If you are entering a driveway which has no place to turn your only options are to drive in forwards then drive out in reverse, or reverse in and drive out forwards. The ideal way is to reverse in but you must only reverse in when the driveway is on your left unless you can wait in a median strip for a safe gap to reverse across to your right.
While you could hold up traffic when reversing to your left, you can control the speed of the traffic as you slow down and, with clear indication of your intent, will be able to perform the manoeuvre. You may have to pull over and wait for a gap in traffic to reverse complete the manoeuvre, but that gap does not have to be as big as if you were reversing out of the driveway. You can follow the steps outlined in the Road Code for reversing into a driveway.
If you are reversing into your driveway on the left, it’s likely that you will be leaving the driveway in the opposite direction and will benefit from being able to pull out forwards across to the other side of the road.
One of the problems with reversing out of a driveway is that you will have massive blind spots. The rear (C) and middle (B) pillars of your car plus your passenger seat and passenger create large obstructions preventing you from seeing up the road. There will often be lamp posts, bushes, pedestrians and parked cars that make it more difficult. You will have to be careful of pedestrians using the footpath.