Originally Posted by
Cymro
You have a choice of north vs. south.
The service offering is the same. North is closer to (north) security, so easier to access; south is larger and can feel more private when it's relatively quiet (but both can be a bit of a zoo if rammed).
If the plane is going from a 'B' gate then there is also a business lounge on the 'B' concourse, which is small and quite pleasant and makes for an easier boarding experience. However LHR announces its gates very late so it can be hard to get the best out of it.
As the OP is in J when departing T5 and will therefore have fast track access I would suggest this is a clear cut decision - north fast track!
I have been using north fast track quite a bit over the last few months and it is rarely busy and often I have no wait. It helps that is it set apart from the general north security entrance, and far away from CW/F check in where I guess most will go to south fast track.
If coming from the tube or HEX then north is a no brainer. If connecting and using the landside routing I often go via north fast track too. I know it is a longer walk there and then to the south lounges, but I know I will not have a wait at north fast track whereas I probably will do at south. When I come up in the lift I try and look across to south fast track and if the line is up to or past the bp gates then north it is.