Well there were people and animals on the ground, and planes and heli's are not meant to be low over parks and rec type areas like this one was. It didn't appear to be doing a TO or landing. Very frustrating that the camera didn't get it, but what can ya do ? :/
Check the CAA regs for the exact wording. Large gatherings of people, you cannot overfly, you must fly around. A few walkers on the ground, like I saw from your drone, means you just need to skirt around them when flying lower. The area you filmed, is open country and that means you are allowed to fly lower if you wish too.
With that said, altitude is a pilot's friend because if you suddenly have an engine issue, you have plenty of altitude to lose as you sort out the problem. So for the most part, smart pilots prefer to be up there, it just gives you more time to sort out an engine issue or setup for an outlanding if the situation of an engine out, occurs. Down low, you only have seconds, 3,000ft up, you have a good while to get sorted.
I am from Wales and when I lived back home, I would often fly my aircraft close to the ridge of the Brecon Beacons. Obviously not super close, but there were often walkers as I flew a hundred or two feet above and off to the steep side of that ridge. That was perfectly legal to do, despite what anyone on the ground might have thought.
Take a look at the UK CAA regs and you might find that the pilot you saw, was doing nothing illegal.