The British tanks came in the 8.1 patch of World of Tanks as the only one with a medium tank at tier 1. Only the heavy and the medium tank lines are yet put in the game, but in the 8.2 Patch, there is a chance that the artillery and tank destroyers will come in too. But we still aren't sure. Don't worry about the light tanks though, they exist but only at tier 1,2 and 3. After those tiers you will have to move on to the medium tanks and afterwards to the heavy, unless of course you choose to go for the other line where it exists from only mediums from tiers 2-10.
These tanks have a way different gameplay then the other countries, as for example the germans like to go in fights and absorb all the damage while doing lots of damage. Or the russians that like to build manoeuvrable tanks to flank and cover behind the big ones. The British tanks have alot of manoeuvrability too but you will still like to play more aggresive then the russians, since they have a very high penetration and low reload speed. That allows them to have a very high DPM(Damage Per Minute). Their big weakness is their armor, it's almost like it's not even there. The FV215B (tier 10 heavy tank) Can easily be penetrated by most tier 6 tanks.