Beware - if you are very shortsighted or have a complex prescription (like -10 or more) then the more reputable online retailers don't seem to go up to that strength.
I found one that did - but they had some alarmingly bad reviews & by the time I'd factored in getting high index lenses made as thin as possible, the cost was still about £70 just for lenses. (Can't remember name of the website but they were on MoneySavingExpert advertising glasses for £5 all-in.)
£70 for possibly badly fitted lenses was too much of a risk for me, in the end I went with an independent optician who quoted me £200-ish for lenses that I knew would be fitted perfectly.
If I had a much lower prescription, and was in the market for a cheap spare pair I'd consider Glasses Direct, from my research they seem to be amongst the more reputable.
But do consider an independent optician -these days they can be very conscious that they have to price-match with the chains & online. (My presciption is so complex that I am never eligible for any of the 'free pairs' that Specsavers etc offer so I'm not missing out on those).