The main reason that commercial training is more expensive is because the instructors with the right bits of paper to teach to commercial level are in shorter supply than PPL instructors, and usually have enough experience to do other better-paid aviation jobs, so have to be paid a proper salary to retain them.
PPL intructors on the other hand are usually hour-building towards an airline career, and so will instruct for (almost) anything as long as they don't go too hungry.
It's simple market forces. About £12ph of the PPL rates goes to pay for the instructor, who will get a salary in the £6,000 - 9,000 region. About £36ph for a commercial instructor will give a salary of £18,000 - 27,000.