The moment you realise your dog is missing is the time to get out there and start searching. Don't sit back and wait thinking they will return, this can waste valuable time and gives them the chance to get further away. Walk around your local area calling your dog's name, they might not have gone far.
In a word, yes. Since 2016 all dogs in England, Scotland and Wales are legally required to be microchipped and their details held on an authorised database. This has been the law in Northern Ireland since 2012.
The Control Of Dogs Order 1992 states that any dog in a public place must wear a collar with the owner’s name, address and postcode engraved or written on it, or engraved on a dog ID tag attached to the dog’s collar.