Just to clarify the position on lost dogs. They are taken to a local authority dog pound by the dog warden. If scanned they can be reunited with the owner before even entering the stray dog system. If not scanned, they will enter the pound (and go and take a look at your local one if you haven't before, and say honestly if you'd want your dog to spend any time there)and once there they have SEVEN days. After seven days the local authority may legally dispose of the dogs either by rehoming or permanently via the needle. Approximately 10-12% of dogs entering the system will face death. Usually the ones selected will be the ones less easy to rehome - older dogs, less pretty ones, ones that are stressed in kennels, Staffies, lurchers and greyhounds (so common another one will be along in a minute). If the dog is rehomed, and you then see it walking down the street with its new family ten days later, you will have no legal comeback. This happened recently with a family who went on holiday for a fortnight - by the time they came back their unchipped family pet had escaped from neighbour looking after it, had gone to pound and been rehomed - not a thing they could do about it.
Most responsible owners already chip. I'm heavily involved with rescue and I know incidents where the following have happened in homes where owners are devoted and wouldn't think to let their dog stray.
Dog peacefully at home. House burgled - dog kicked and beaten by burglars, dog escapes through open back doors and garden gate.
Dog escapes while workmen in the house, not closing doors/gates.
Dog let out into garden for ablutions. High wind has pulled down part of fencing - dog escapes.
If your dog escapes and is killed on the road, around half of local Councils will scan dead dogs for chips where possible so they can notify owners. It's the worst news but at least it brings closure.
Not everyone who finds a stray dog takes it to the dog warden. Many will take it to a local vet especially OOH on weekends. A vet can scan a dog quickly thus setting in train alerting an owner swiftly. If you lose your dog you can alert the chip company so if anyone scans and this chip comes up it's flagged as a lost dog.