I don't think, legally, the previous owner ever loses title. It wouldn't be theft if you, or the police etc, haven't deliberately deprived them of the car. Making reasonable efforts to find and ask them covers that. Difficulty proving "intent to permanently deprive" is why we have "taking without consent" as a separate offence - so a good idea to square it with DVSA/police first.
One common work around over the title is to have a counterclaim of equal value: "You're only in a position to ask for your car back because I've had it in safe keeping inside my garage. The cost of that safe keeping is at least equal to the value of your abandoned car, so I took the car as payment in your absence." I suppose, at a push, they could pay you in cash and take the car back. You'd be on a weaker footing if you didn't have to deal with the car, better to have the consent of whose land it's on so you can argue you were acting as their agent, paid in knackered Volvo.
So I could take a car, any car, lock it up in my garage and a year later tell the owner he owes me storage fees.
Possibly but only if you had a reason to move/remove the vehicle and possibly a contract with the owner (verbal or written) for storing it.
I think the local authority's and police have different rules but even a private landlord renting out a garage has some things they have to do before they can dispose of a vehicle.
I used to work at a garage where the landlord rented some small units to private individuals who were repairing or storing cars, one had an MG in it and after several months of them not paying or visiting the garage he emptied the unit and sold the car to one of our customers. After a few weeks (presumably the new owner tried to get a V5) the original owner went to the police and the new owner had to give the car back and lost the work he had since put into it (fortunately not much but he had got it running and fitted some new bits). The person who had bought it got their money back from the landlord but I seem to remember that the problem was the landlord hadn't waited long enough or given notice to the person renting, it was some technicality.
Its a mine field although I know its tempting when you look at abandoned vehicles, I have seen quite a few with local authority removal notices and thought what a shame I could do something with that.