It's mainly from an insurance stand point, but i meeting recently with the Head of Training, he said that he would have no problem with family and friends going on training flight depending on what is requred on the flight.
If the training manual states that on this flight you are ment to carry out forced landings, stalls ect you will not be able to take people with you, only other cadets.
I am a NZ cadet so i'm unsure if the rules are a bit different (other than by defult UK guys don't get a PPL).
If you get thru and are a cadet, just ask someone. the forgivness vs permission model doesn't work well at CTC