people drive untaxed cars that are not registered in their name, so they don't get any letters or fines from DVLA. They drive along with no tax until one day they are stopped in a roadside Police check, and then they are nabbed.
The cars I reported were removed the same day. As far as the Police were concerned, they were not taxed, and illegal....Engine numbers (on the engine block) can help the DVLA trace owners, but these are often filed off (also an offence)
Also because the police have localised purges, because they haven't hit your area yet doesn't mean they aren't going to. Apparently in some areas a lot of folks convinced it'd never happen to them got a bit of a surprise when it did.