101 police non-emergency number. Tell them vehicle abandoned with keys in the ignition and been there some time. That will log the incident. Take keys, drop keys at local police next day. If owner turns up with baseball bat call 999.
I would not do anything to move the bike. It is not your responsibility that it has been left unsecured. By notifying the police, recovering the keys (a valuable) and handing it in to the police saying you found them near a vehicle reg xxxxxxx, you have done the right thing. The police wont recover an abandoned bike if it is legally parked taxed and insured, just as they won’t impound your bicycle that is worth £1500 that has been left unsecured outside a newsagents. By removing the keys you make it more likely insurance will pay if the bike is stolen, by reporting the bike you protect yourself if you are stopped and have the keys. Once you start moving the bike (other than to stop it being a danger or an obstruction) you open a legal minefield that could see you inside.
That reminds me. I have been in this situation. My first contact with LB was seeing Keti knocked off by a pedestrian that ran into her in 2006. Got medical help via a helpful bus driver who radioed in for the emergency services. Paramedics came - took Keti to hospital all before the police arrived. I moved her bike off the road, and gave keys to police and she recovered them from the police and got the bike picked up. I signed up here as a result. I had an unsecured bike and the keys, but am sure I did the right thing not trying to resolve this myself.