Don't worry about using them they will certainly be using you and you will know it after some of the hours they will ask you to put in!!
If you are in the regions then I would say most of the offices would react quite well to receiving a letter enquiring about cadetships whilst studying or jobs at the end of your degree. The London offices tend to be a little harder to get into and follow the rule book (ie: apply iva website) mainly due to their size and volume of interest. Check out all the websites for the big firms and you should find out background information as well as contact details etc.