It depends on where you are going to do your PhD (inside london or outside). I study outside and am currently on £12,300 per year, for inside london its £14,350. This increases anually. Usually for home students (UK) the studentship includes a stipend (figures quoted above) and payment of tuition fees. It does sound strange that your department is asking to apply for funding if you are being covered by the BBSRC award. However it could be that it is for extra funding. I would definitely ask at the department/graduate office to clarify.
Hope this helps.