Also, direct airside transfers (those passengers who are in transit and have not 'entered' the UK, would not be able to use that lounge as it is essentially in the UK and outside the immigration restricted zone
Not entirely true, transfers are allowed, although the current procedure is for the passanger to be escorted by a security agent and then handed over to the handling agent at the gate before departure of the connecting flight in T3. This is an issue that was highlighted by the DfT and was reported in the national press a few months ago.