It is often said that stolen dogs have their ears cut off, but the only ACTUAL cases have in fact been greyhounds that were disposed of by owners (to save vet fees) and did it to avoid detection.

The main difference is that if property (and a dog is property) is permanently marked (in this case a tattoo is a visable mark), then it MUST be returned to the owner.
There is no legal requirement to scan 'property' and as a microchip is not a visable mark, it does not count under property law.
Obviously that doesn't mean that a dog won't be scanned, but another thing to bear in mind is that there can be problems with scanners not reading microchips and chips moving and not being pickedup. I'm not sure of the exact details but there is also a 7 day window where a dog in a council home becomes their property to do what they want with if the owner is not found. Because a dog is considered 'property' and by tattooing you are 'marking' that property, the window is increased to 28 days which is the standard time marked property has to be kept