If you download or write away for all the application materials, it will have a table in it of in which gathered field each course accepts applications. Mostly it will say something like 1,2,3, or 2,3 or maybe in your case only 3. When Admissions receive your application,they hold it over to the first/next applicable deadline. So, if Socio-legal only accepts applications in GF3 then they'll hold it over until the January deadline. They then send all the applications received to the departement in one pack.
In other words, no applying early won't help you if only 1 field is applicable, just apply in time!
(P.S. Apply for Hertford! It has some of the best Graduate accomodation in Oxford and its one of the cheaper rents.)