The T means they are buit with a Trent core (HP System). They have 24 fuel burners as apposed to 18 on a non Trent core. And produce 60,600 Lbs thrust as opposed to 58000lbs thrust on the G.
It increses fuel econ and lowers harmful NOx gass emmisions,
The can of course be rated to G or H when fitted to the 747-400 by means of the FAFC rating plug. BA has trent core engines but most if not all are G rated. Im an avionics chap. No doubt someone will give a more lengthy explination.