I've had this from the Electoral Services Officer at Sheffield City Council on the nature of tomorrow's voter registration deadline if you are walking up with a filled in form:
Whilst we would prefer them to be received during the day so we can get them processed the legislation says it's 12 midnight. Anyone coming after our reception closes at 5 will need to hand in their registration form at the Porters Lodge entrance round the back of the Town Hall. Postal Vote applications have to be in by 5pm.