Bradford has the University which has an Optometry Department. That makes the number of locums in the area quite high. If you work in places like Leeds, Huddersfield, Bradford or Wakefield you should expect rates of £250 weekdays and £275 on Saturdays. The area gets booked very quickly and it is not easy to get many bookings in advance. Last minute ones are often available but you should not rely in getting locum cover that way.
If you are willing to travel a bit, approx. 60 miles around Bradford area, you can expect to get your rates being increased. BOOTS usually pays £275 weekdays and £325 on weekends but sometimes they can be a bit unreliable and you can get last minute cancellations. Bookings are usually 25 min with them and contact lenses and children 20 min.
Specsavers , Asda or Tesco only offer 20 min appointments. In some towns, due to the unemployment level and social problems, the number of referrals you can expect in a day can be higher than in other “wealthier” locations. That is something you should bear in mind if you work for companies with shorter testing times.
I hope that helps.