I work part time at a Specsavers Branch. Each Optician can charge their own rate for an eye test, most charge between £15 and £25.
The price of contact lenses depends on what sort you have, but on average its £22 for a months supply of dailies. You can set up a direct debit with them, and they will give you certain extras and money off eye tests, depending on which branch you go to.
As far as i know they'll accept most prescriptions as long as they're no more than two years old, once it reaches two years old they'll require you to take an eye test with them before continuing to supply you
Hope this helps.