That shop is not a legitimate iPhone reseller. From the looks of it, many people have complained about its dubious practices and service.
https://www.yell.com/biz/lowesmoor-mobi-tech-worcester-8026125/
Again, it is IRRELEVANT that the phone worked. It could have been blacklisted after you activated it.
Your best bet is to RETURN the phone to the store for a refund or better yet, take it YOURSELF there. Do not send it!
The other option is to take it to an Apple Store for service. If the phone is blacklisted and reported stolen, Apple will probably not service it as you are not the legitimate owner.