You are, but at LHR, arriving and departing passengers are in two different streams and do not mingle, so you need to cross from the arrivals stream (where you come in) to the departing stream (where the lounges are) and to do that you need to go through both a security check and a boarding pass check. You will not pass the boarding pass check because as you have been told twice already, you have to have a boarding pass for T5 to be able to access the departing passenger stream.