Does anyone know of an opticians that is happy to put lenses in frames you've bought someplace else?

I would like to buy some designer frames online, they are MUCH cheaper than if I bought them from Specsavers etc(they'd cost over £125 and they don't have the particular brand i'm after) however, my sister who bought some lovely frames online went to a couple of opticians including Specsavers and they weren't prepared to put lenses in frames she'd bought elsewhere even though she was happy to pay for the sight test and lenses. There are lots of sites these days that sell designer frames for a much cheaper price compared to the high street opticians, however there is one wee problem which Optician(s) is prepared to put lenses in them for you?