I think that it is unlikely that you would be able to transfer to Law from History. If by some chance one of the Law undergraduates at your college dropped out at the outset of the course, leaving a space available, and if you could satisfy the Law Fellows of your college that you would have been admitted to read Law if you had applied to do so, then you might be able to switch courses, but I wouldn't bet on that occurring.
Please bear in mind that adding one extra undergraduate to a cohort of, say, ten adds significantly to the teaching commitments of the academic staff, and in many colleges the cohort will be smaller than ten. The Law undergraduates take Mods at the end of their second term, and that's another reason why transferring in from another subject would be unlikely.
In addition, there is no indirect route into a subject which is one of the more popular subjects applied for (approx nine applicants for each place). In fairness to all applicants, colleges can hardly allow much or any swapping of subjects.
Changing subjects between different humanities used to be easier, but things have changed. In the early days of the Experimental Psychology course at Oxford, it was a refuge for overwhelmed Chemists and Physicists, but that has changed too. Nowadays it's more a case of stay in your lane.
If you are interested in becoming a lawyer, you can study law as a postgraduate subject, by taking a PGDL course (one year) or a senior status law degree (usually two years, and available at Cambridge, IIRC). It appears anecdotally that the degree most commonly held by lawyers who do not have Law degrees is History. It's a subject which provides a good preparation for becoming a lawyer, especially a disputes lawyer. Litigation is a form of historiography.