Sorry to say you are doomed, no matter QR's best intentions they are let down by MANchester airport, oh and the fact that at least one of their flights a day is the "Virgin Australia" flight.

TL;DR - you may have priority, but the security is rubbish, the boarding is rubbish, so it doesn't get you into the best mood - and QR pay for the best lounge at Manchester, but you aren't going to see much due to the general incompetence of the airport. And you are unlikely to get QSuites on the plane. Once you board QR can start to do things, but it's still on sub-premium planes.

Look forward to the return trip more:
QR28 ex Manchester always late
QSuite Future Availability MAN and HKG
Qatar to buy/lease ex-Virgin Australia 777-300ERs


Context, I grew up with MAN as my nearest airport, so I have reasons to return, but would in general prefer to arrive in MAN (they can only mess it up so far, though enjoy watching your priority bags arrive last, or at least in the middle), but would proactively rather leave from London. Last flight out of MAN was as follows: Arrived 150 minutes prior to flight, the time was spent as follows: 30 minutes at check-in, as the belt stopped working and everyone was confused. 30 minutes at "priority" security as they managed to jam up the bags with their super sensitive but as a result super pointless extra checks (whilst the staff belt ran just fine next to us), got to the lounge for ten minutes ("final call was made so left") before we arrived at one of the furthest gates to hang around for another 30 minutes as the crew boarded around when we got there*. At least we then boarded, some 30 minutes after departure time. QR made up the difference by dropping us straight into the departures lounge at DOH for our onward connections.

*This beats the 30 minutes I once spent in the space between boarding and getting on the flight, as they preferred us to be a bit closer to the airbridge.

So, enjoy that "J", it will make it only a little better than "Y" until you actually get on the plane and get that first Rose Champagne. Sorry until then. But, to be helpful, my tips for minimise/maximise would be:

1. Arrive super early, to just remove the stress (note, you probably can't hand over luggage more than 3 hrs early)
2. Stay in the lounge as long as possible (it is actually a nice lounge and has great views, btw as a business class passenger you should really get priority entry, the people queuing have priority pass and similar)
3. Board as late as possible... there is obviously a risk/reward to this (I normally prefer to be on the plane as early as possible. MAN isn't the place for that)
4. Once onboard, then you can enjoy the service as QR really wanted you to.