From the little I know, it appears quite a big and growing 'scene', just search on YouTube for 'bike life uk', or see https://www.vice.com/en_uk/article/moped-gang-raise-it-up-uk-ace-cafe-london There a often big meet ups near Tower Bridge / Potters Fields. Often these kids have no plates and wear balaclavas. It's a big problem and only a matter of time until innocent pedestrians get killed. The police appear unable to stop it, and won't give chase for fear of causing an accident. The kids therefore behave with impunity. We've seen some huge 'rideouts in London' especially around Halloween.l, with bikes riding on pavements through Red lights in the wrong lane