Seen at the bottom of my garden in Surrey. Large bird, approximately twice the size of a wood pigeon, preening with a straight yellow beak. Brown feathers. Also stayed for several minutes and the beak was wide open.
Difficult to know without even a bad photo as a record shot. Very often when birds "sun" and preen they can look a lot more fluffed up and larger from a distance so my instant thought (hasten to add that I am no expert ! ) was perhaps a female blackbird. They often open their bill (or beak) as a way to cool off during hot sunny days and they will stay for several minutes whilst they "sun" I would wait for other opinions if it was indeed definitely twice the size of a Woodpigeon; do you have a pond or other source of water nearby ?