I tried to see if there was a similar arrangement for the Plaza Premium arrival lounge at LHR T2, but couldn’t find one so I sent an email to them asking about making a reservation. This is the reply (as helpful as can be, I suppose!):
Thanks for your interests in our service.
Priority Pass cardholders are entitled to basic lounge use in Plaza Premium Lounge at London Heathrow. No reservation is needed. Card members are entitled for Basic Lounge Use of the Lounges such as hot showers, all day food & beverages, internet access, laptop plug, comfortable sofas, International newspapers & magazines, International TV channels and secure left luggage area. Private resting area is not included.
As we are just service provider, for accompanying guest charge and usage limitation, please kindly contact the card issuing company for more details.
As instructed by Priority Pass, for further enquiry on charges, limitation and card entitlement, kindly contact Priority Pass directly.
Should you need further assistance, please do not hesitate to contact us.