A common whitethroat (Sylvia communis), photographed in the park by DKG in the summer of 2019. It is probably either a female or a juvenile; the male is more distinctively coloured.
The whitethroat is one of those species of tiny frail looking birds that migrate immense distances. They winter in Africa, south of the Sahara, and return to Europe in the spring to breed.
Whitethroats are very territorial and choose open country with bushes and brambles for their nesting sites; they sing loudly from a high perch somewhere inside their territory.
Here is a video recording of their song.
Video by british-garden-birds.com 2016