There is little alternative to the train for travel to central London, meaning that trains have a captive market, but in outer London and most of the rest of the country, car travel is mostly viable. There are definitely people who would like to use the train but don't because of the cost.
Not only is car travel more viable, it is more convenient. There will always be people who will use the train if it were cheaper. However there is a balancing point between catering for these people (which may be desirable) and encouraging ever longer commutes and/or generation of trips, (which is not so desirable).