Kinda like when Menezes ran to board the tube you mean?
Slightly different wouldn't you think? If a car with blue lights is following you for a while most likely they want you to stop. Also you're on a moped and the police car wasn't after you it would quite easily over take you.
Menezes wasn't challenged for one. In a place with heightened terrorism alert and on a system where terrorist attacks just took place. Yes there was police incompetence but let's not pretend a high street scumbag on a ped in your local high street is comparable to that. After all I don't know many undercover ARU that get the call out for moped crime.