There is really no problem simulating 'real' RT congestion. All that is needed is a suitable tape inserted into the scenario. However, most RAF sims do not have a suitable tape because the 'system' does not produce one.
The 6-hour airborne IRT for the VC10 must be a prime example of where money could be saved by use of the simulator. Hate to think what the taxpayer was shelling out for each IRT.