Welcome to Flyertalk and welcome to the BA forum. Hopefully you will find this place useful for planning your travels.

You get lounge access at INV (small Aspire lounge at the far right) and at the 3 Galleries lounges in Heathrow. North, South and T5B. With so much time you may want to visit T5B, it is often the most relaxing lounge. If you do decide to do this, read up on the Passageway Guide for Departing passengers, in the BA Forum Dashboard. If you don't take the transits on your visit to T5 you won't go wrong, and the distance is only a few hundred metres anyway. There are also lounge guides in the Dashboard.

When you land into LHR from INV there is a flight connection channel there, with no security, so you can be off the aircraft and heading to the lounge very quickly because you started at a domestic airport. However you can indeed instead bear rightwards, and follow the exit signs to get out of the airside area quite easily and quickly. For someone from the Highlands, "fresh air" and "Hounslow" don't often go together, but if it's a smoke break then this would be the time to do it. Getting out of airside usually isn't that easy. Returning inwards is ok, but as you note, there would then be a security check.