I am not sure of the numbers but you can get a feel by looking at the commissioning graduation lists published in the Daily Telegraph Court and Social. It lists graduates by rank, branch, and degrees.
A significant number of pilot graduates have BEng. A significant number dstined for the Admin Sec have LLB. There is a not insignificant number of acting pilot officer graduates too. I would hazard a guess at 60-40.
The non graduate (ie school leaver) brings youth and ability whereas the unversity graduate brings experience. The youth is often more malleable than the graduate.
If you can impress at OASC, improve during IOT and absorb what is taught at FTS you will have no problems.