A flock of Black-headed Gulls on the lake at Tredegar House.
No interactions present.
This would be a very unusual time of year to see a flock of Little Gulls like this, although they are seen year round. As Chris suggests, adult Little Gulls hae pale wings above, dark below, but don't show the black on the primaries above. As their name suggests, they are genuinely little, too, much smaller than Black-headed.
My Flickr photos...
I'll have to swot up on my bird spotting skills.
Lat/Lng: 51.5636, -3.0339
OS grid ref: ST284854