I'm only somebody who passed the course, not somebody who runs them, but as I understand it the course provider puts together something to meet the learning objectives and gets it approved by the CAA (or presumably other national authorities) - that includes the training provider's judgment of what is required to deliver a candidate who can pass the skill test.
This webpage seem to cover the minima very well.
Where I did mine at Denham the minima were 30hrs groundschool and 3 hours flying + the skill test; I believe that the groundschool is the minimum in the regs, but if you add in a couple of "ridealongs" I did in the back of a C172 to observe an experienced instructor at work, I did closer to 6 hours and even that was working stupidly hard.
The 30 hours for me were more than enough - but that's me, with a CPL, and formerly a university lecturer teaching aeronautical engineering. I think that anybody with "only" a PPL and neither higher aeronautical knowledge nor prior teaching experience, is going to really struggle to get ready for their skill test in that time.
The one thing that would have made it easier for me (and I was working stupidly hard) would have been to get more right hand seat time recently during which I practiced flying accurately from that seat and I did mention that to them in the end of course debrief as something that I think Denham (or any other course provider) should advise future course participants to get.
G