The only exception to this would be if you use company transport to get to work, and are then expected to change, as you then have no choice as to what time you arrive. For example I used to work on an army base, and we had to change when we got there, but we also had to enter the site on a bus provided by the site, so we were paid from the moment we arrived even though we then got ten minutes to get changed. If you make your own way to site I would say you are meant to arrive early enough to change then present yourself at your post at the start of your shift.