The choices are all a bit random and you have often to guess or shoehorn your job into one of their options, but it makes a big difference.
22yo was down as working in catering, which he does. Bizarrely, one of the options is Assistant Manager (what does that even mean?) which he also is. Changing it saved £170 
Out of interest I had a look at mine. I am down as a Bursar which is what most people would recognise my job as, but my job title is Director of Finance and Operations. Change it to Finance Director and my premium goes up £40!
Obviously you can't lie, but there is a significant grey area for most roles.