Is remarkably easy.
My bike was locked up at waterloo station and I lost the key to my kryptonite U lock. its an entry level lock, but they're tough motherfuckers to get open. So I spent an hour with a hacksaw cutting it apart, and NO-ONE questioned what I was doing. In fact, 2 members of the public actually came over to help me. Bear in mind this was at 6pm on Saturday at one of London's busiest terminals. I had informed station staff of what I was doing, and they had no problem with it. In fact, they offered me a pair of huge pliers for the job.
i'm not really sure how I feel about all this.