I used Derek for PPL and IMC groundschool, he is very good and I would recommend him. There is no harm in self studying though if that is your preference, after all, knowledge is power so to speak.
In my opinion though, Derek's tuition is worth every penny. He is very experienced and there is definitely an advantage with one on one tuition.
For me it is a no brainer as proven by the fact that I went back to him for my IMC groundschool after having completed my PPL and got some hours under my belt.
Hope this helps.