Ok I have just booked flights for next august in from NCl - LHR, LHR-MIA and back on the same PNR in club class, does this mean I should get access to both the NCl lounge and the LHR lounge when I return from MIA and am waiting for the LHR-NCL flight?
I take it you are booked in Club only LHR-MIA and NCL-LHR is Y only? Or does the connecting flight to Heathrow have a Club cabin?
Either way (and especially if the second option is the case) I think you should be able to use the BA lounges in Newcastle (is there one?) and Heathrow before and in between each flight.
However, until a more knowledgeable BA person comes along (remembering we know more about VS here) then the Flyertalk BA forum may be a better bet for a clear answer.