"I notice that coach prices are considerably less than rail."

One can't really make that assumption. W/ rail, a LOT depends on how far ahead you book, your ages, the time of day you travel, etc.

Generally a coach will be cheaper, but often not by much. And sometimes a couple of £ savings isn't worth the extra time a coach takes. Other times (like when you've waited til the last minute to book) the train can cost many times what a coach will.

W/o knowing more about you/your travel dates/etc., one can't tell which is best.

" sometimes a couple of £ savings isn't worth the extra time a coach takes."

And sometimes, as with Heathrow-Oxford, the train's a great deal slower, messier, pricier, less frequent, infinitely more likely to be suffocatingly overcrowded and goes from a more inconvenient start off point to an even less convenient part of town.

There are really only two generalisations it's possible to make:
- there are no generalisations possible: you need to check for yourself which option, properly analysed, meets your time, cost and real flexibility needs.
- the anti-bus bigotry so common in America is hopelessly irrelevant in Britain.

Which said, I'd never in a million years get the coach from Oxford to Chester. You need to play round with options on the National Express and National Rail websites, but you'll see that Advance rail fares after 0930 (which you can buy up to 1800 the night before travel) are only trivially pricier than the bus for a far, far more frequent service that takes only half the time. In this particular case, the train journey is prettier - though that's not routinely the case.

Same goes for Portsmouth to London.

In a coach you're stuck in your seat. On a train you can get up and walk around if you feel like it - and there are more loos. And personally, I find it much easier to read on a train than on a coach.

For myself, I'd only get a coach for a journey of not much more than an hour, or if I'd left it so late that the train was ruinously expensive.

For Portsmouth to London check the train prices on the Megabus website

" On a train you can get up and walk around if you feel like it"

On some trains.

Between London and Oxford it's easier to walk round the double-decker coach, you're certain to get a seat on the coach (whereas on some trains you're likely to have to stand than to be able to sit, making most reading impossible), and you get free end to end wifi (and a power connection) on the bus, but only intermittent connection through a dongle on the train.

The passenger:toilet ratio is identical between most long-distance buses and standard class carriages in trains.