I am in Lincolnshire so quite near to you! It depends on their experience, if they are bringing any clients with them, The quality of their work, the speed they work at ie: one tech can do an excellent set of nails in 1hr 45mins another may take 2 hrs 15 mins. Thats a diff of 30 mins when you could earn a manicure or another service.
You have to consider all angles.
I would do a basic pay rate per hour plus commision that way the more she does the more she earns i have pm'd you the rate i pay.