Some changes were made recently.
No longer T4 was made a long time ago.
All GGL are allowed two guests flying BA flight into any lounge with the possible exception of CCR.
If one holds a CCR (or has the right to one), then their name normally appears on the list of eligible lounge guests for that day, and they are entitled officially (though there are some differences of opinion on this) to take a guest flying BA into the CCR. I've already used the CCR at T5 when flying CE, and also guested someone flying UK Bus.