I don't understand how the police haven't caught up with them yet, by sheer luck if nothing else.

Vauxhall spares can't be worth THAT much surely?

Well, if your bumper gets stolen and you have to use a bodyshop to replace it, there's a potential for a scam. Bodyshop pays someone to steal various parts. Owners claim on insurance and use said bodyshop. Bodyshop replaces bumper (maybe even the one they stole) and gets paid for the job despite doing nothing special.

Vauxhall - maybe because it's easier to steal/strip bodyparts, or they're a more popular vehicle (or both).

Given the driving skills of the average Corsa driver, they might not be expensive, but they are in great demand.