I'd say it's a great idea and you should go for it. Be prepared to spend many hours though putting together the content. Taking onboard the whole course is far too much and will take years (by the end of which the specifications would have changed!)
So think about starting with 1 subject and moving on from there. So far the ones I have come across (apart from the free ones) are Times tutorial and myGCSE.
Times tutorial costs £99 a year covers english maths and science with every topic includes a small intro and interactive questions and answers but it's not board specific, very outdated and not well organised.
myGCSE is board specific and uptodate, includes videos explaining the subjects, room for notes, practice questions and answers. The videos are brief and great for recaps on subjects. Cost £50.
If I was you I would start with something similar to myGCSE for maths. You can do the popular board first (Edexcel, and may be add aqa later on). Do brief videos for each topic (that would be around 120 for Edexcel and include practice questions and answers. Maybe even add past paper questions per topic section. For something in that range a cost £50 to £70 / year would be fair.
Remember that it's the parents you are selling these to and £50 is nothing in comparison with a £15000 year private schooling.