I have no experience of taking eye test from boots opticians. And i have just search it their sites and find that they signs''Get a full, professional eye check for 10 pounds at Boots Opticians or D&A The Opticians, saving 50% on the usual price of $20.'' Besides, i also find her calls online 0845 125 3752. Maybe you can call them and consult them what you want to know.
I did the eye exam at boots opticians last year for free during the national day. That day there was the discount activity that buying a pair of eyeglasses may send you the eye exam for free. About the exact price of eye exam, you could call them and ask the clients there. Or you can wait to see whether there is the discount activity this year.