If you completely privatise transportation services, costs will go up, and so usage will fall

If usage falls, companies lose money, so they cut back on costs, reducing number of vehicles and lines, so overall reducing the amount of public transportation. If failing companies are acquired by bigger ones, monopolies may be formed.

Most importantly, privatisation would have an outsized effect on the poor. Those who cannot afford alternative travel, will be forced to pay higher costs, and so be at a greater disadvantage than they are already at. Those who are more wealthy will be able to afford alternative travel, and likely find it cheaper in the long term, even if there are greater short term costs. This would further increase the wealth divide in the uk.