It's pretty common to sell something for $1, at least in the US, because of laws about gifts vs sale or something. This is just funny because it's a pound shop being sold for a pound.

Edit: I was a bit wrong https://en.wikipedia.org/wiki/Peppercorn_(law)

The seller thinks the value is net negative to them. The buyer thinks it still has a potential positive value. Both would agree to just hand it over.

Unfortunately, UK law does not allow that. Consideration must go both ways. The simplest way is to sell for the minimum reasonable amount. $1 is traditional in the US. In the UK it is £1. The other commenters link has a good writeup on the practice.

They broke from "everything is a pound" years ago. They also still do a really bad job making it clear what is no longer a pound.