Are you looking for the Tekkeon MyPower GO, i.e
Then you'll have better luck at electronic shops like Maplins
While they may not have the Tekkeon Product itself, you can easily find similar products. They are normally in the portable power section. Its also often called the emergency power section.
Maplins has a number of of them.
Argos has one as well.
And you can often find a really cheapo version in the Pound Shop's, which while basic, usually do work.
If you want the most options however, I suggest you stick to Amazon UK and search for Universal batterys / Emergency Battery's.