I go to specsavers for they eye test. I have varifocals, so the buy one get one free becomes really expensive, because you don't get the lens for free.
If it's not been 2 years since your last eye test, i would recommend getting a pair using your current prescription. There is a very good reason for this. Every time you increase your prescription and get a stronger lens, your eyes weaken and you need stronger glasses, so going early will actually cause your sight to deteriorate quicker
Ive used glassesdirect quite often for an extra pair. The starting price can be very good and the quality is great
https://www.glassesdirect.co.uk/