Additionally to say that training a lawyer (by that I assume you mean the three year university course of Jurisprudence, which is merely a degree and no where near the full training, which commences after university) to say that a degree at Oxford where classes are individual, or in tiny groups, is not as expensive as teaching in huge groups in lecture theatres is simply wrong. Oxbridge is our most expensive method of teaching at university level because of the numbers.
Like for like, it will absolutely cost more. But the differences between the cost of people studying say, Engineering, Biochemistry and History is that the engineers will have lab equipment to test things like centrifugal force and other concepts beyond my ken, specialist software and so on. The biochemists will need chemistry labs and animals (with associated staff), chemicals and costs of trials and so on, and the historians will need books. There may be better examples, but those come from my immediate experience. Clearly, a History course is significantly cheaper to teach, whether the classes are large or small.
I am not going to argue about the difference between jurisprudence and full legal training - it doesn't matter to what I am saying, which boils down to the fact that course costs are not equal, although fees are, and one subject subsidises another. I think that's fair, as otherwise there would be expensive courses that are only available to those who can afford them. Medical degrees are extremely expensive, and the only reason I mentioned it in my post was in response to you saying that doctors and nurses pay fees like any other students. They do, but their training is massively subsidised.
IMO that is as it should be, as the country needs them, so that subsidy is an investment in our collective health and well-being; but when companies who have not invested a penny in the training (towards which we have all contributed) cream off the fully qualified staff to treat patients who can afford high fees, and then refer them back to the front of NHS queues when they need further treatment, then IMO it is taking the proverbial. I can see no reason why those companies should not be asked to pay a levy towards training the people needed to replace the ones they have 'poached'.
My son works in the private sector, and when he moved from one role to another his employer had to pay such a levy, as the previous role involved seconding staff to clients, and the deal was that if such staff were 'poached' it was at a cost of 6 months' salary. Earlier in his career he was sponsored by an employer through postgrad professional exams, on the understanding that if he left their employment within a set period he would have to pay back the costs. That is not an unusual scenario in the private sector, and I don't see why it should be different in the public one.