The Police no longer take in strays or have anything to do with them EXCEPT those which come under the Dangerous Dogs Act. All responsibility was handed over to the local councils about 2 years ago.
Long story short, Councils don't HAVE to have an out of hours stray service. Please ring your council and find out if they do. If not, they will give you the option of either taking him yourself to the nearest (usually sole) "reception point" for their county and will advise you of where it is or to take him in yoursef until the dog warden or whoever they have doing that job is back in work. If you took him in you CAN if you wish keep him instead of handing him into the council dog warden but by law you MUST inform the council that you've found him, you MUST keep him for at least 28 days and you MUST endeavour to find his owner and reunite him if possible - but as you say, you are (understandably) unwilling to do that. The other option is to do nothing.
Yes, it's utterly inhumane, potentially dangerous to the public (road accidents and possible aggression) and a fucking disgrace - thank the Labour Government).
If he's a large bull breed and is old he will stand no chance in a pound and will almost certainly be put to sleep as soon as he has been there for the legally demanded 7 days. The pounds in your area are particularly shit and barbaric - Fleet pound in West Yorks admit that they don't even try to find homes or rescue places for GSDs, Rotts, Staffs or other bull breeds or most large breeds but they immediately kill them as soon as the law says they can - ie after 7 days. Springfield, which has just lost it's license and which Fleet has taken over from, were abusing the dogs in their "care" and killed them in huge numbers.
There is another option which is a bit grey when ot comes to law but is the best, most humane and most sensible IMHO although it isn't going to be easy. It's to catch the dog (spare lead, jeans belt?) and, before or after, obtain a place in a NO-KILL rescue for him. That will mean you explaining to them that you're aware of your responsibilities in law but that you're equally aware that he will probably be PTS if he gets taken to the pound and ask them if they can care for him instead of you. If they have a good working relationship with the local council dog warden staff they'll be able to swing it and keep him with them, eventually reoming him if the owner isn't located within 7 days. BUT... rescue places are already like rocking horse droppings and no rescue wants an elderly large bull breed as they know that rehoming him will be virtually impossible and that he will take up a rescue space for a bloody long time whilst being a potential vet bill on legs. (You can thank the "Oh I'd never adopt from rescue/Oh I'd never own a bull breed, they're dangerous" mentality of half the UK population for that).
If you can catch him that would be the best thing you can do... but PLEASE be sure that you are asking for help from RESCUE with a NO-KILL policy and NOT from a pound simply calling itself a rescue.
If you need transport to get the dog anywhere, speak to Suzanne at Yorkshire Animal Ambulance (details online) - she owes me one! Tell her that her rescue contact from Poplar Farm Kennels passed you her way. :) And, get back to me if I can help.
HTH.