Yes as per the 2nd para of my post you need to be on a course. Check out the link below for the list of approved flight schools. Lasors has all the rules and regulations on exemptions.
If you are looking at "savings" from the overall cost of flight training then the ATPL theory is generally not where they lie. Most people save some money along the way by hour building overseas or even buying shares in aircraft in the UK.
Approved FTO's