You need to do the BPTC (Bar Professional Training Course) after the GDL in order to become a qualified barrister, you cannot become one with the GDL alone. There is also no need to do a Masters if you don't want to, you can get onto the GDL with a decent degree (usually a 2.1 or above). The BPTC lasts a year full-time, as does the GDL.
Many solicitors and barristers do not have an undergraduate law degree, so your degree in forensic anthropology won't put you at a disadvantage as far as I know.