Originally Posted by stewardo

I don't understand how you propose to physically get to T3 lounges without a boarding pass for a flight departing from T3? And then to get from T3 to T1?

Heathrow isn't like a US airport - when you are in the departures area (where the lounges are) there isn't a big sign saying "Exit & Baggage Claim", or an easy route to other terminals. Changing terminals in that situation would involve interacting with security staff and potentially border control.

Heathrow operator BAA require that passengers only access the airside areas of the terminal they are departing from - that is enforced by security checkpoint and lounge staff.