Encouraging people to buy tickets the day before would be a simple option. Modern ticket machines do of course have this option... just a couple of snags! Firstly, it as far as I know they only allow it at the origin station, and secondly it can only be done after a certain time (is it 6pm?).

Hmmm... ticket machines may be ok in larger stations - but as with the recent stolen information point displays - machines on unmanned stations are very vulnerable.
At the end of the day, we're still talking about using coinage - and at least the guard can give you change.

Perhaps a dedicated 'ticket for tomorrow' machine for SVB stations could be installed at BRI^» (Bristol Temple Meads - next trains) platform 1!

I've noted quite a few shift workers boarding & alighting the train at St Andrews Rd and Avonmouth both morning and evening. I'm guessing they get an Open return? A quicker on-board punch ticket system could still offer the same service.

I seem to remember that some of the Valley Lines booking offices just involve someone with an Avantix^▸ (Ticket Issuing System used on board trains), and I think one of them is/was an arrangement with the station taxi office (I'm thinking back a few years - not sure if it still happens). Perhaps local shops could be given Avantix machines

I guess at the end of the day it's down to profits. Wouldn't a local shop want a cut for the service they provide? There are also shops that aren't open before 8 and after 5pm. 

I'm not too sure about the cash only thing - depending on how black and white it is - I'm all for encouraging passengers to pay small fares in cash, but for longer journeys and irregular travellers who wouldn't know about the arrangement, it just seems to make things too difficult and off-putting

Well as the Severn Beach Line is a halt (for want of a better technical term) - the local people who use it (at least from SVB to BRI) could be told of the changes by leaflet (in fact, I had a timetable put through my door recently - even though it's due to change in May). Day trippers (*cough!*) going to SVB are either twitchers, spotters (EWS^▸ (English Welsh & Scottish Railway Ltd, now known as DB Schenker Rail (UK^▸ (United Kingdom))) 66's at Avonmouth), older people wishing to reminisce about the boating lake and the strip clubs - or people who are madly curious. In that event - people travelling from the BRI end could use the booking office if they hadn't change (obviously there are flaws with every plan).