There are obviously fixed costs like tax , not necessarily insurance anymore. My previous insurer was pay per mile. However you have oil , you need to replace tryes , brakes , shock absorbers and plenty of other things more often . My other halfs car is a three year old one litre. Just cost us near a grand for new brakes and tyres.
Insurance for a year is about the same as a single family return ticket to London. Tyres are about £50 a year. Total cost of a car on PCP with petrol at £2 a litre works out at half the price per mile as the WCML for 1 person, and crucially is far more useful - went bowling yesterday, 8 miles each way, can't take the train, there's no bus on Sunday, there's no realiable taxi service either, just the car.
When we used to have two cars the car cost £1500 for the year, for the capital cost of the car, the petrol for the entire time, insurance, tax, etc, it's the cost of having two cars, which based on where we were and where we needed to be was required that year. The equation will be different for people living in London of course.
For the majority of the country the majority of the time, rail has to compete on the marginal cost and benefits, 81% of people have access to a car or van, so to use the train has to be a better option.
The big tickets - the long distance peak time tickets like £350 return Manchester to London - are presumably way down, and that will really impact on total revenue, you need to sell a lot of £50 leisure tickets to make up for that.
Many season tickets were always massively cheap. Rugby to London on Virgin for £9k a year for example, or about £40 a journey, compared with £150 for a day return, so loss of revenue there isn't a major factor on the revenue-per-journey side even if they now only take off peak returns at £50 per day that still increases revenue per journey, but the occasional business traveler no longer sees the need to be in London for 10AM.