That most council pounds won't put to sleep after a stray dog's 7 days are up (UK, 5 days in Ireland) is bullshit I'm afraid. It depends on various things; the pound's policy, the amount of space in the pound vs the amount of new dogs coming in and even the dog's age and health. Some council pounds go out of their way to avoid PTS, but sadly many don't. This is why I and others lile me work to get dogs out of pounds and into rescue.
My extensive experience has taught me that some pounds will PTS if a dog is deemed unlikely to be homed - aggressive, ill, old, ugly, for example, all cases I've come across. In some pounds simple kennel cough is enough to warrant PTS. It's easily treatable but spreads like wildfire and pounds have been known to PTS a dog suffering from it rather than go to the cost of treating him and running the risk of having to treat all the dogs in the pound. Thank God it isn't Christmastime, which is the worst time of all for poundies.
May I advise that you email the website DogLost and ask them to include this dog on their site. DogLost is the main website serving this purpose and very successful. Also post on Facebook and ask others to spread the word if you have it, ask your local rescue to make note in case of enquiries and/or post details on their website and put posters up in local pet shops, vets and in or outside schools where they'll get maximum passers-by reading them.
Report to the Police -although strays are by law entirely yhe responsibility of councils now, in effect a dog is a possession and so you can report him as found. (Police might huff at this but you are in your rights to insist that they note it in case someone reports their dog as lost to them).
You might also like to have a word with Gladys at ProDogs Direct in South London, who will know where else to contact. Equally let Wood Green Animal Shelter know as well as Battersea in the hope that the owner might contact them.
If you hear that he is at risk of PTS and that after 7 days the pound won't keep him please contact me here (put Vallhala in the title cos I'm a bit dopey about missing things!) and let me know and I'll endeavour to get him into the safety of a no-kill rescue.
Finally, well done you for caring. So many people wouldn't.
HTH.
Val