As a Qantas Club member flying on a BA flight you will have access to the BA Executive Club lounge (Terraces Lounge) in Terminal 1. It is quite a nice lounge with shower facilities.
If you are a Qantas Platinum FF you can also use the First Class lounge which is to the left of the reception desk. The Terraces lounge (business class lounge) is to the right of the reception desk.
As you are arriving from Terminal 4, the normal procedure is to follow the signs to Flight Connections. That will take you down to the lower level and onto a bus that will take you to the Flight Connections centre between near terminals 1, 2 & 3. When you get there, its up the escalator and through security where you will find the transfer desks if you need to get the connecting boarding pass. If you already have the BP, just follow the signs to Terminal 1.
Note that when leaving the Flight Connections transfer desk area, you do a U turn and go down the escalator. Chose the correct escalator or you will end up at the wrong terminal!
Follow the signs to the airline lounges. The BA lounge is at the far end of the large open area you will walk through, surrounded by shops.
Make sure you have your Qantas Club card with you. The BA lounge agents are notorious for denying access if you don't have the card, even if you boarding pass clearly identifies your membership of status.
If you are travelling in business class, you can also use the FastTrack lane at the security checkpoint at the Heathrow Flight Connections Centre, but will need to show your Club Europe boarding pass. Otherwise you will need to join the regular security queue, which can be busy in the mornings. Sometimes it can be quicker to actually clear immigration at Terminal 4 as if arriving into the UK and then take the Heathrow Express over to Terminal 1 and go back through security.
Also note that when returning from Paris to London, if you are booked on the Qantas Codeshare of the BA flight you will not get access to the BA lounge at CDG since that lounge is operated by Air France for BA and is not a real BA lounge and hence no reciprocal rights for Qantas Club passengers. If you are on the BA flight number you may get into the lounge but be prepared to be turned away. I will be through there in a few weeks and expect to get access since I will be travelling on the BA flight number in business class as a OneWorld Emerald.
Just realised you are going to Nice, so my comments about CDG and Paris are not relevant to you. Enjoy nice - it will be nice at this time of the year. Got to love the coarse "sand" on the "beach"
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