Don't want to fall fowl of mentioning competitors but the only UK places at the moment would be Overclockers, the place in Bolton that rhymes with flan (think barcodes), who both appear to still be doing demos.
And then 'computer planet' in Reading, Leeds and TCR London where doing - and still may be - doing demos, along with the well know 'posh' shop in Knightsbridge.
Would be worth contacting them first to check before planning any trips.
Outside of that it would be to go to one of the gaming or tech shows that run throughout the year, or check a few forums and see if anyone in your local area is kind enough to do a demo.
edit: should have clarified that the shops listed are for Vive, not sure about the Rift.