I read somewhere about a zero rate contract. Paying only for hours worked. From my understanding, the argument is, and bare with me, i might start woffeling:
From the start of the first job (lets say 6am) to the end of the last job (12pm) is 6 hours. So you need to make sure that they are getting at least the minimum wage between that. If your paying £10 an hour for 4 hours work, and the rest of the time is travelling, then you dont have to pay them for the time they are traveling because the rate of pay is still above the national minimum wage for the 6 hours.
Now Im not a professional, but i think ive interpreted what i read quite well.