Around here (in Suffolk) the majority of stations are unstaffed but all of the rural lines have conductors on the trains. (Some mainline services are 'driver only' trains but all of the stations they serve have ticket facilities). It is, of course, fairly easy to do a 'short hop' before the conductor comes round (even if you're not trying to avoid paying your fare) but the train company must have calculated that the cost of putting ticket machines in at the small stations (and then servicing them) would be greater than the additional revenue that they'd bring in.
In practice, most passengers make return journeys and will see the conductor on at least one leg of their journey. Since the return fare is often only slightly more than the single one, the loss to the rail company (from only receiving payment in one direction) is actually fairly minimal. For example, when I go to the pub on a Sunday evening I should pay £2.85 return. I often don't see the conductor on the outbound journey (so I don't have to pay) but it still costs me £2.60 to get home again, so the loss to the rail company is only 25p.
The National Fares Manual (which is the 'bible' for rail ticket sales) groups stations together for longer journeys. So, for example, if you were travelling from a rural station near to Swansea to a rural station near to Ipswich, you'd be charged the Swansea to Ipswich fare. However local journeys have their own fare tables, so you should be charged different fares for different journeys within a local area.