A car service I use waits for me by the Hertz Car Rental counter. It is slightly beyond the exit area where the crowds waits for arriving passengers so it is much easier to spot the driver with the signs.

However, if you want to use the lounge, the alternative is to tell the driver to walk into the Virgin Revivals Lounge to ask for you at reception. Just tell the reception people that you are expecting a driver around the appointed time. No wait going back out to look for the driver outside the lounge. You can comfortably sit in the lounge and read a paper.

90 minutes might be a bit rushed if the plane is a bit late (VS has trouble getting gates at LHR on landing from time to time) if you want to truly want to enjoy the lounge. Would say 2 hours, then you could fit in a massage even if there this an available slot. 90 minutes is the bare mininum time.