CCTV recommendation (POE)

Autopilot

Most of the smaller specialists, like the one you linked to, make thier money from consulting and installation. If you are looking to self install, Amazon is as good a place as any, with stress free returns. If you want to pay for a company to install them I would find a well established local firm.
As for what else you will need will depend on what you want to achieve, and also what camera you are buying. Some cameras record onto a memory card, some have a PVR (hard drive), some upload to the cloud.

Ian_H

Looking at a CCTV system myself at the moment but I think I'd avoid Amazon/Ebay for Hikvision gear and stick with retailers that use guaranteed UK sourced stock to avoid any warranty / configuration issues.  Google search shows quite a large price range from legit suppliers and I suspect it's possible to haggle when it comes to placing the final order.