Sorry KK the vid does not seem to have anything on it and only lasts a second m others may have more luck.A long single note whistle may be Starling as they make a variety of sounds
This is a duplication of another thread Bird song identificatio [https://community.rspb.org.uk/chat/f/hello/278730/bird-song-identificatio], which I'll share my, possibly unhelpful reply.
It sounds almost like a curlew flight call, but the immediate surroundings in the clip don't match a curlews normal habitat.
You could try the following link, noting what the habitat is around the area in question is. https://www.british-birdsongs.uk/