I've seen the same species of bird several times over small lakes in Buckinghamshire but I don't know what it is. I've not seen it in detail as it's always on the move. It's a similar size and shape to a lesser black backed gull with fairly long, slender wings and the colour is a whitish grey. It hunts for fish by constantly flying back and forth or in figure of eight shapes over the water about 10 metres above the surface and flying at quite a fast speed. Every so often it swoops down for food from the surface of the water or from just under the surface, it doesn't dive into the water.
The most distinctive feature is the way it quarters back and forth over the surface of small to medium size lakes at fairly high speed which it will do for fairly prolonged periods of time - I've watched it for 5 to 10 minutes just constantly criss-crossing the same lake over and over again.Most times that I've seen this bird it has been a solitary individual
I'm not a great bird watcher but I'm familiar with most common British birds and have spent a lot of my life near water (including sea) but I don't recognise this particular species