Owning a mock police vehicle and a uniform is not impersonation:

In the US and Canada it appears to be perfectly legal to make a car look like a police car and to own a police uniform so long as you don’t actually use them to convince people you are a real police officer.

If people are looking at new laws, it may make sense to regulate ownership of these items to prevent a similar shootings in the future. The UK has much tighter laws about police uniforms:

Any person who, not being a member of a police force or special constable, has in his possession any article of police uniform shall, unless he proves that he obtained possession of that article lawfully and has possession of it for a lawful purpose, be guilty of an offence and liable on summary conviction to a fine not exceeding level 1 on the standard scale.