By the time you get off the plane clear security and get to a lounge you may be looking at 4 hours to kill rather than 5. I think that is probably too little time to break out of the airport especially as traffic hits the morning peak.

Why not book yourself into the spa ASAP when you arrive to get the best chance of a treatment (avoid the chair) then go to Plane Food and buy yourself a decent breakfast before spending a couple of hours in the lounge?

The Spa and better lounge are located at the South end of T5. The Spa treatments are a little poor I think but it does give you something to kill time.