If budget is paramount, I think you can obtain the JAR-PPL on touring motor gliders, and then do the differences training to be able to fly other single-engine piston types. The advantage is that motor-glider training can be had for as little as £60-70/hour, and will provide nearly all the skills you need for more conventional SEP (and a few you don't need also).
There is a club in Oxfordshire, that I know of, that may be suitable.
I see no problem starting the ATPL ground school reading early, though its probably a good idea to get the PPL exams out the way first. There isn't a time limit on completing a distance learning course. A provider should be able to sell you the training material, and credit you later when you officially register for the course.
You need to have passed CPL or ATPL exams before starting CPL flight training anyway.