Originally Posted by
Wiirachay
I re-read your question and I assumed the lounge was land-side. I can't answer about being able to exit terminal 3 for a second time. However, isn't there a land-side shuttle connecting T3 and T5?
You can also check if the carriers have interlining agreements. That way, you won't have to claim your luggage at all in LHR. I suggest you keep things simple and just buy a pass to the BA lounges, if they permit.
There is airside access between the terminals with the FCC (Flight Connection Centre) -- I think that is what you meant. One could also go between the two terminals landside as well, there is a HEX link into T5.
I agree with you, best to keep things simple and to access a lounge in the terminal the OP is departing from. I would also suggest the OP has checked luggage interlined so that it doesn't have to be claimed at LHR (assuming an interline agreement is in place between the first carrier and BA).
It's been a couple years since I've been to LHR - I purposely avoid it - but I believe British Law requires inbound and outbound passengers to be separated. Some of the terminals at LHR aren't the best designed for this, but I know they've been making upgrades at LHR (and even LGW) to stay in compliance with this law.
Upon arrival I believe the OP would either have to proceed directly to the FCC (Flight Connection Centre) or proceed landside via Immigrations/HM Customs.
To me, it sounds like a lot of hassle just to access the VS lounge. If it were me, I would either hop on the HEX and go into the city and/or get a day room at a hotel near the airport. If the OP wants a lounge on departure, I would suggest looking into obtaining access at a BA lounge.