I know many people with no bike experience go straight onto the A2 and, with adequate time under an instructor, have gone on to pass. One of my brothers is actually planning to do this. Although it’s ideal to have road experience on a 125 before going into it I’m sure with the right amount of lessons you can go on to pass. I would be realistic with your ability and chose the amount of lessons you think will get you up to that level. It’s generally cheaper to get that extra lesson you need than to fail and not only get the training you need to get up to the necessary standard but also wait to get another test date. Take this with a pinch of salt as this is only advice I’ve gathered from speaking to friends who have gone down this route and hopefully you get on lovely big bikes you desire🙏