United Kingdom Passport Photo 35mm x 45mm requirements

Recency and Condition: Photos must be taken within the last month. They must be in color, clear, in sharp focus, and free from "red-eye".
Background and Lighting: The background must be plain cream or light grey. There should be no shadows on the face or behind the head.
Pose: You must face forward and look directly at the camera. The image should be a close-up of your face, head, and shoulders.
Expression: A neutral expression is required with the mouth closed; smiling, frowning, or raised eyebrows are not permitted.
Face and Eyes: Eyes must be open and clearly visible without hair across them. Nothing should cover the outline of the eyes, nose, or mouth.

Printed Photo Size: Must be 45mm high by 35mm wide.
Head Size: The distance from the bottom of the chin to the crown of the head must be between 29mm and 34mm.
Print Quality: Photos should be printed on plain white photographic paper by a professional studio or booth; home-printed photos are often rejected for quality issues.
Digital Photo Technicals: For online applications, the file must be between 50KB and 10MB and at least 600 pixels wide by 750 pixels tall.

Glasses: It is highly recommended to remove glasses. If worn, frames must not cover the eyes, and there must be no glare or reflection on the lenses. Sunglasses and tinted lenses are prohibited.
Head Coverings: These are only allowed for religious or medical reasons. Fashion headwear (like beanies or headbands) is not permitted.
Objects: No other people or objects (such as toys or dummies) should be in the photo.

Age 5 and Under: Children in this age group do not need to look directly at the camera or maintain a neutral expression.
Under 1 Year Old: Babies do not need to have their eyes open. If a baby’s head requires support, the supporting hand must not be visible in the frame.

References

Create passport and visa photos online.