youtube.com/watch This is the sound of Whooper Swans. Now if this had been during the Autumn/Winter period I would have suggested that or Bewick Swans. Even though there are a small number of nesting/breeding Whooper Swans of around 22 pairs in parts of Northern Scotland,, but numbers do vary during the Spring/Summer period every year, up and down.. If you could have sent a recording of the sound as well as what part of the UK you heard that sound, that would have helped. Sound and pictured of Whooper Swans in link.
Regards,
Ian.