Sent a complaint to rockstar lol.
Will post reply when I get one.
Basically told them that I think it would add to the game massively if they allowed you to store and modify police, vans and utility vehicles.
also said that I thought the game was based around heists and chasing the almighty dolar, so wouldn't modified vans and police transporters be a good idea for getaway vehicles.
And I thought that the game was supposed to be a game where you could do what you want, when you want but they tell us we can't do what we want to do.