I never take eye test at tesco opticians and don't know how much they will charge. But i saw a sign at their sites:"call 0845 6013479 to book a FREE sight test appointment". So, i think you can call and might be get an eye test free of charge. Hope this will help you. Good luck.
My sister had the eye text for free at tesco opticians because there is the activity that buying a pair of eyeglasses with full price may give the eye test as present on the National Day. So I don't know the exact price. You can call their store and ask in detail from the client. By the way, you can also ask whether there is the discount activity now.
As my research on the internet, I have skimmed through the website of tesco opticians. It says that you can get a free eye examination by calling 0845 601 3479 to book an appointment. What's more, you can buy different glasses and service at tesco opticians, such as prescription glasses, contact lenses, designer sunglasses and corporate eyecare. The price for the products and service is variable. If you want to know more about it, you can check http://www.tescoopticians.com/default.aspx