Originally Posted by
Majuki
I agree with this. However...
If you are arriving on one of the one stop security flights, you would go airside after arriving at LHR, which would provide direct access to the T3 lounges. There are several ones you could access, but I'd probably only consider Cathay Pacific or Qantas (in addition to the Admirals Club and BA lounges). There are some caveats with this approach since the airside transfer bus from T3 to T5 has limited hours. You can read more
here. There is a way to go back landside at T3 without being escorted if you miss the airside bus to T5 and do Flight Connections, but you need to watch the time since getting between terminals can take some effort, especially if you're unfamiliar with LHR. Do not consider this unless your layover is 3+ hours.
If you're not on a flight with one stop security, then you'd not (easily) have the option to go airside at T3 for lounges.
What is a one stop security and how do you know if your flight has that?
This may or may not be the same thing but last month arriving @ TPE on JX from the US I went right into the regular airport area airside and went on to the lounge and my next flight with out any security. That was a first for me.