The lifetime of a £50 camera, SD cards and battery pack would probably be on average a matter of a few weeks or days.
Remember the £480 per camera probably also includes costs for associated systems and training to actually transfer the data off the camera (scaled to however many officers in each shift have a camera), store the data redundantly (imagine what the response was if the crucial video was 'lost'), systems to view, locate and extract the video for court/interview purposes, provide large scale battery charging etc.
Given a basic GoPro seems to be about £150 there is obviously some government contact premium here but it isn't as bad as it could be.