For a budget of £100 max the old 6x9cm folders might be ideal in terms of size of neg but all of them, I think, have fixed lenses which means that head-shots involves the photographer being so close as to be in the subject's face.
It is sad but true that in MF £100 will not get you much, if anything at all in terms of interchangeable lenses. Big enlargements of full body or half body( waist upwards) cropped for head only, might work OK but unless the print is very big a 35mm camera with a 90-120 lens and say Ilford D100 film might do as good a job
Having said all that I have a 645N which at the size of prints I make might not make much sense but for some reason I just had to have it
Mike