Most DIY sheds (and to be honest most electrical type places, even if aimed at trade) sell Copper Coated Alumium cable, aka CCA
Indeed go to trade places like CPC, Neweys and Denmans and you'll not be able to buy anything else but CCA.
Your local security counter will sell it too, but will always offer many other pure copper alternatives.
CCA is frankly horrible tat and I, and *most* decent installers IMHO won't use it on a proper job.
However, it sounds as if - from the description, whcih might be wrong - screwfix sell proper 'copper' cable and TBH at that price it should be.
If it is, it's a good start in terms of ease of wiring (stripping cable) and zone resistance.