The FAA website provides various free publications. Worth a look. The link below will take you to a handbook covering all ppl ground subjects. Well written and illustrated in glorious Technicolor (download as pdf). Slightly “American” but the Principles of Flight are the same on both sides of the Atlantic – except the CAA will make things much more complicated and charge you a minimum of £200 for doing so.
Pilot's Handbook of Aeronautical Knowledge
Next link also covers ppl syllabus
Welcome to the free online private pilot ground school
Third link will generate mock exam papers in each subject. £20 for a year (unlimited use). EASA syllabus – not American
AirQuiz - Online Practice Examinations for Pilots