They probably live on the Roehampton estate which is pretty much the only even slightly dodgy area anywhere near the park. As for CCTV coverage mentioned upthread, pretty sure there is approx zero in the park, except maybe at the ballet school.
WTF is going on with these people? I might get flack for suggesting that the system is not capable of dealing with these low lives.
I would be happy to see karma do it's job as they disappear under the wheels of the nearest bus - as long as the bike escapes undamaged.