So I take it that its not possible to make Huge Cities for Brittania?

My first campaign was with the House of Julii which I completed very easily, I am now playing as Brittania and have almost wiped out the Gauls but am a bit dissappointed that even though Londinium is at 15000 population it won't allow me to upgrade it any more. Indeed all the towns I hold are over 6000.

Just wondering if there is anyway of getting further upgrades to Brittanic cities, was gonna try and send a spy into Rome but he died enroute, is there any point in sending another one? Are spys even capable of learning military secrets from other factions?