vent gap

Hi all, expertise needed! In the hunt for causes of heat loss, I've just discovered something odd. Our 2007 brick-cavity house has two 9x9" exterior air bricks which have square, terracotta ducting going through both wall 'skins' and butt up roughly to the INTERIOR plasterboard wall. Inside...

I have debated installing loft insulation...... My house is a 4 bed detached and i pay £100 per month for gas and electric. Joists are 70mm so putting insulation in between joists is futile and impossible as thinnest width i can find is 100mm. And i am not stupid enough to start raising...