Once life at University starts, academic achievements become an important part of university life for any aspiring barrister. Traditionally, you need at least a 2:1 to stand yourself in good stead to apply to the Bar. University grade boundaries vary greatly from those you will have been used to at GCSE or A-Level. 

3rd = 40-49% (A pass)
2:2 = 50-59% (Lower second class honours)
2:1 = 60-69% (Upper second class honours)
1st = 70+% (First class honours)

As well as your exam results, it is also crucial to get involved in extra-curricular activities (which are brilliant fun anyway). Activities such as mooting, mock trials, debating or any other form of public speaking are particularly useful for the advocacy skills that you will no doubt use as a barrister.