My employer is with cyclescheme, and my understanding of it was that you can choose whatever supplier you want, depending on if they are listed as participating on the cyclescheme website. I did everything through that website, rather than through the shop's website.
All you need to do is go and get a quote from
Ribble, punch the quote, and your details into the cyclescheme site, and then they'll produce a voucher for you to redeem at the shop. The voucher comes through the post.