Access to LHR Terminal 3 oneworld lounges when flying BA from T5

Hello, I am flying on BA out of Heathrow Terminal 5 to Paris in early January and wanted to check out the Qantas lounge in Terminal 3. I believe there is an airside bus connecting the terminals, just wondering if anyone knows if you can access T3 when your flight departs from T5 at Heathrow?

I can't see why not but allow an hour to get across to T5 as the bus and associated queue as well as T5 security can take a while.

My understanding is that Heathrow security won't let anyone departing on a flight from one terminal access any of the other terminals.

It's a pity, for the BA First Lounge in T5 is nothing special, and is often crowded and very hot inside, with bain-marie food, and drinks that are generally not cold.

Seems it can be done, if you're connecting:

"Once you land at T5 go to flight connections and get the bus to T3, clear security. Enjoy T3

You can only do that with a connecting flight, not just to check out alternative lounges. If you have a connecting flight and you do make it to T3 it is also worth checking out the excellent Cathay Pacific Lounge there.