If you have a problem with your eyes (for example, if you feel you can no longer see as well as you did at your last eye examination or your eyes are sore) the you can ask for an earlier appointment and this would be funded by the NHS. Unfortunately, the NHS will not pay for an eye test if you just want a new pair of glasses and you think your prescription is the same as it was at the last eye examination. So there may be a charge in this situation.
Your optician may also wish to see you sooner than the recommended testing interval if they wish to repeat some tests on you or check up on an eye condition. In these cases your examination would be paid for by the NHS.