Unless you live and work in London/Birmingham/Mancs etc, public transport is not viable. It wasn't before Beeching had the right idea, so most definitely isn't now.
Most bus services outside of Londonium etc seem to be reducing, and aren't integrated into the rest of the public transport system.
For example, for me to get to Central London purely by public transport, its:
20 minute walk into town
Catch the bus, Tuesday at 11am
Run from Bicester bus station in order to catch the 12:56 choo choo, or walk, and wait nearly an hour for the 13:54
Hour* in to Marylebone
30mins on Bakerloo and Central to St Pauls etc.
So to get to London on a Tuesday, its 5hrs, assuming I want to arrive at 3:30pm.
The return journey is more fraught, as I have to wait for the following Tuesday for the bus.
Or I can take the car, about 2.5hrs, and cheaper even considering the congestion and ULEZ charges that the Mayor needs to pull down statues.
*Used to be around 40m, but every time there is a hint that HS2 might get scaled back further, HS2 Ltd's owners, Network Rail, who also run the timetables, extend it back out to an hour, in order to make the Birmingham to London routes 2 hours, rather than the 1hr15 it used to be.