Moderator, Iberia Airlines, Airport Lounges, and Ambassador, British Airways Executive Club
Join Date: Feb 2010
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There are a number of paid for lounges airside in LHR, and with the exception of T5 Aspire you are allowed to go to them via Flight Connections to use them. It may not be easy to get back to your correct terminal, and you may need staff assistance, but as a matter of principle, this is allowed and facilitated.
You can't do this to access the oneworld lounges since by and large they are told to restrict access to those deparating that terminal that day (various exceptions have been reported, I don't believe this to be the SOP however). The one possible exception, for Admirals own membership arrangements, would be Admirals T3 since these operate outwith oneworld policies in that situation