Age 21 no qualifications and want to start studying to become a soilicitor/barrister or bank manager?

First work out which you want to aim for. Look up the qualifications needed and maybe start with better GCSE grades and move on to A levels. You could potentially do these at evening class and carry on with your building job to make money in the mean time. It is never too late to start studying and now that you have decided you want to get yourself a better education, your motivation will be good.