IMO, all the reasons given in this thread for -not- granting lounge access to the OP in LHR are correct, if it were not for a little known clause in the oneworld rules which clearly suggest the opposite:

"...Customers flying in Economy Class on the domestic or intra-European segment to connect to or from a oneworld longhaul First or Business Class flight can also use the lounge applicable to the class in which they are travelling on the longhaul segment."

The full text can be read on the OW Electronic Timetable, by clicking on "Information -> Lounge Access" from the Main Menu.