Envoys; first off, adding a tee ot a run is an absolute no-no. adding a Y would still be a bad thing but it may streamline the air a tad better. i hope I don't upset the cart but from the pics i see you have a rookie diy installation,.in other words whoever butchered this mess should have shot and p....d on.
In order to get some supply air up into this attic (loft you should have a seperate run, perhaps 6" diam terminating to a 4x10 or4x12 register. Get rid of that tee and keep the bathroom on a single run with a damper in it so you can "throttle the air down somewhat to the bathroom. Cut in a new 6" diam run off the plenum with a damper and run it up into your attic /loft.
A lot will depend on your furnace's rated CFM and btu output ....you may have to increase your rpm slightly and or 'balance all the other existing supply runs in order to throw more air the attic way. And you will absolutely also need some RETURN air from this attic/loft, in order to get some decent supply air up to it.