"Professional" termination of mains wires
I see those ferrules all the time on the lights I get from those friendly east Asian purveyors of sensibly priced goods. They are supplied in brown/hot and blue/neutral, which are Europe and UK color standards.  

European use screw terminal blocks to terminate wires, so those ferrules work rather well with them. They also increase the effective wire size somewhat. You would particularly expect to see them on products with fine-stranded wires such as used in cordage.  They are basically unheard of in the United States because they don't play well with our wire-nuts.  

It doesn't surprise me if UK DIY shops (like our Home Depot) don't sell them. They are kind of a specialty item.  You'd have better luck with a proper electrical supply.   

I would expect those ferrules to take a special crimp tool.