Could it be a red eyed Vireo probably unlikely as it comes from North America, as we only get around 3 or 4 a year.
or a Green Warbler ? or a wood warbler ? ( or another kind of warbler ? or something else ?
Sorry about the quality of the photo it was in the distance
Thank you Tony, PB, and Robbo, Brilliant I have not seen one before, just a shame the photo wasn’t a better one, but hopefully I might see it again and get a better photo.