Indeed, as mentioned , the terminals are separate and lounge access is limited to the terminal from which you are departing - this is , I believe, also a rule imposed by Heathrow Airport

Even if it was permitted, one other potential issue is that would need to be departing in 1st or Business class to use the lounge ( you do not mention class of travel )

With 8 hours , I would try and get out for a few hours rather thann sit in the terminal