I really do hope this is a fake post, there is so much incorrect and shit advice being chucked around.
I've worked with dogs for over 20 years, in rescue, foster, training and behaviour, with dangerous dogs, seized dogs and advising courts - @Feraldogmum I am plenty grown up enough thanks and am fully aware of what goes on, and what doesn't. I am also up to date on current dog related law and am not discussing issues from a decade or more ago as if they're recent!
You do have to report to the dog warden that you have found a dog, or you're at risk of theft by finding and you're potentially preventing someone getting their dog back. If you opt to keep the dog you must take reasonable steps to locate the owner and the dog will never be unquestionably, legally, yours (unlike if the dog does 7 days in the pound and then you take them on.)
I hope you never experience losing a dog to theft or escape and the endless worry, until well after their natural life span must be over, about where they are and what happened.
It is likely that a dumped dog has been dumped by their legal owner but it is also very possible its been dumped by a thief, for myriad reasons.
UK Pounds are not euthanising dogs who are fit, healthy, happy and have homes asking to adopt them.
UK Police are NOT involved in stray dog cases unless there is a dangerous dog issue.
UK Vets are not confiscating dogs from anyone, they have no power to do so and frankly have better things to be doing with their time.
(what goes on in other countries I have no idea, it isn't relevant in any case).
As I say, I hope this is a fake post and I also hope fervently some posters here NEVER find someone elses straying, stolen, dumped dog and simply decide to keep it based on outdated ideas and misery-monger bullshit!