I have recently been through this. I needed new frames, so chose the ones I wanted in Vision Express (£240!), then found them online for £90.
I looked at ciliary blue, and the thinnest lenses for me would be £95 (-1.74 index).
I went to Specsavers for a free eye test, and whilst there they quoted me £90 for the -1.74 lenses, with a £35 fitting fee. Given that I got a free eye test with them, I thought the extra £35 fitting fee was fair enough so I went with Specsavers.