Firstly, after recent changes Gatwick Express is now part of the Southern franchise and many of their trains run through Gatwick and to the south coast, so they will probably still be fairly well used.
Heathrow and Stansted express are probably operating more or less empty at the moment, but what do you mean about "help[ing] the movement of people"...? What people, going where? Are you seriously suggesting running Heathrow and Stansted express units to Scotland to replace domestic air travel? Even when
BR▸ was "uniform" (which it wasn't) these trains would never have been mobilized in an attempt to replace flights. The GEx trains are third-rail so moving them outside the southern region is a non-starter. Here are the other problems:
1. All essentially outer suburban commuter stock. No fun being stuck on one of them for 4-6 hours*.
2. Drivers with relevant route knowledge would not have known the traction and vice versa.
3. Somehow you have to get all these units from where they normally operate onto a different route for which they may not have gauge clearance
and here's the real killer...
4. The situation could resolve itself fairly quickly and with very little notice. Not sure people trying to travel to LHR, LGW and STN once flights start up again would be too impressed to find that all of the rolling stock for the rail connections was half way to Scotland.
Sorry bassett44, but this is nothing to do with the "non-uniformity" of rail privatization, I really don't think you've thought this one through!
It's very easy to say "a lot more needs to be done" or "somebody should do something about this", but that's of no use to anyone unless there is a realistic and practical way of doing something. Believe it or not, the
UK▸ 's rail network is not set up with sufficient slack to leap into action at the drop of a hat if some once-in-a-lifetime event like a volcano paralysing domestic air travel happens, and in my opinion that's quite right!
*pipe down on the Cotswold line please... 
