Hi,
If not offered, ask for the pyjamas even on the LHR-LAX route ( many people change into them even prior to push back)- although If you do this the window in the forward toilet becomes visible to the outside also!
They are of course free to take with you ( together with the slippers).
Ther earlier you get to LHR the more chance you have to get a spa treatment.
The CCR is very nice so arrive in plenty of time to enjoy it ( I arrived 5 hours in advance for one of my flights-stayed at the sofitel and walked across)
Enjoy!!
Regards
TBS