Curious because I regularly take trains all over the UK and have generally found the ones in the south-east and south in general to be the worst, as well as the most expensive, which seems reflected in customer satisfaction surveys at least – http://www.which.co.uk/home-and-garden/leisure/reviews-ns/be...
Virgin Trains, running only long-distance intercity journeys, is compared to London Overground, which runs trains within London every 4-6 minutes for commuters. I'd also guess the average Which? reader is much more likely to rely on a train for work if they live in the south east.
If we compare local services, the south east is much better: much more frequent service, newer trains, generally staffed stations, better value fares within London.
If we compare long-distance services, the trains often start or end in the south east anyway.