I've used corefix and Grip-it for plasterboard.
Corefix are great but only suitable for up to a 25mm gap between the board and brick. Grip-it are strong and will likely hold a modern lightweight TV up no problem if you spread them out and don't have the screen angled too far into the room.
However with such a large gap I'd be cutting a section of the plasterboard and insulation out then building up a solid anchor point from wood on the brick.
It isn't as daunting as it sounds and can be finished over with a plaster skim and paint if you need to bother about cosmetics behind the TV.
Or you could use a simple flat TV mounting bracket held to the plasterboard with Grip-it fixings and a few threaded bars anchored into the brick chemically and then a big washer and nut on the tv side of the bracket. Trouble is, by the time you've done this, you might as well just cut the plasterboard and do it properly.