My take on this is (thinking of central Oxford)... if you wouldn't want to lose it, don't leave bike locked up/ unattended. If you have to, for a good bike: don't use a cable lock, and have two decent D-locks, one through each wheel plus frame (not over top tube), to a secure object. As others, a few hours unattended = 'hack bike', D-locked: rear wh and frame, and prob cable lock front wheel. A friend even had a cheapish front wheel nicked off his bike at swimming pool in Oxford. AIUI, if a thief is brazen enough to use a battery powered angle grinder, pretty much any bike lock will yield, and many decent chains will go to large bolt crops.