I saw this on my yesterday. Above Wormwood Scrubs in London, at 15:56. It's barely visible on camera but you can just about make out a dozen or so white dots that appear high in the sky, well above the height of local helicopter traffic. From my POV they didn't appear to move much in relation to each other, so I thought it could be balloons held in an area with no wind because I couldn't see any wings. But maybe they're gulls with white bellies just hanging out and gliding? I watched for about 5 mins and this is the best my phone could do to capture it.

I wonder if these kinds of things are what people believe are drones or orbs?

the YouTube compression doesn't do it any favors)

But, here's a contrasted adjusted screen shot.

and a black and white cropped section with heavily adjusted levels. The movement is my panning the camera up and down—they didn't appear to move.

Gulls are just mostly all white underneath, classic countershade camouflage in the animal kingdom

Whatever they are they are impossible to see in your YT video on my screen.

From your screen grab they they look just like gulls high up to me

I had a second camera with me, an old Panasonic DMC-LX5. It doesn't have a viewfinder and the screen is cracked so these were the best I could manage, fully zoomed in. You're right they do move in relation to each other, but I watched them for a few minutes and couldn't see any bird-like behaviour.

Here's a hard mix of both photos as layers, showing there were quite a few things there. (only count half the dots).

Yeah, I think this must have been balloons now. They appear a uniform size and some even appear a different colour. I didn't see any movement like birds swooping and changing direction. I wonder if it caused any UFO reports that day.

I know this is an old Facebook post from 2019 but it seems Wormwood Scrubs (which also has a model aircraft flying area) is a launch site for hobbyist balloons.

Saw the see two balloons over Sao Paulo a couple weeks ago. People launch balloons like this nearly every weekend or holiday at parties. It's very common there but I could see how it could be misidentified in other parts of the world.
The balloons are several feet across and the banner below is about the size of a large bedsheet.

I'm inclined to the bird hypothesis, and if there is a fairly strong breeze at altitude, birds could soar while remaining nearly stationary with respect to the ground.

I do take issue with your assessment of the relative heights of unknown objects and helicopters, though. I think you're probably correct, but as a seasoned reader of posts on this site, I'm sure you've seen many cases where observers post wild misinterpretations of size, speed, or distance.

A closer view of one of these "banner lifting" balloons.

Yeah, I think geese maybe. I saw a couple in the same location just the other day. However, when I filmed this I couldn't make out anything other than a white dot (with my eyes) and there was very little movement.