There is the oft-quoted statistic that 70% of train journeys in the UK involve London. That's a lot of the rest of the country to spread the other 30% over. If you consider the relative size and frequency of trains, where around London it's a 12 carriage train every 5 minutes versus a 2 carriage train once an hour, it's entirely believable that some parts of the network see comparatively few passengers.I'm not sure that it does, as that would mean (given that about 40% of the network is electrified) that almost no one uses (for example) the northern section of the MML and the B&H lines, as 80% of passenger use would be on the busiest 50% of electrified routes.
For example if we took London and Southeast as the busiest part of the network that's roughly a one third/two thirds split in terms if passenger numbers across rail sectors, however I'd be surprised if it had less than 30% of the track miles.
In terms of station usage that's likely to be true, however if you cut out 80% of stations you certainly wouldn't be left with 80% of the traffic, as people going to those very busy stations have to come from somewhere.
Network Rail's About Us page says they maintain 20,000 miles of track. Their description of their Southern Region says 3,300 miles of track. So that's a sixth from South of London. I'm not sure what you'd count as London and the Southeast, the Home Counties North of London as well? But it isn't implausible that it's less than a third of the track miles in the UK.