According to the PlusBus website only Bournemouth Interchange (sic) and Poole are designated interchanges, with no mention of Branksome and Christchurch. Perhaps this has changed.
I find that the list of interchange stations on the PlusBus site can be incomplete whilst there may be other interchange stations in the area that aren’t listed. For example, within the Brighton & Hove PlusBus area, whilst not shown as interchange stations on the website, PlusBus add-ons are available from Falmer and Moulsecoomb.
I also made this error, and corrected it in a subsequent post. perhaps "missing" stations were added to the scheme afterwards?
I personally would have used the word local to describe an 8 mile journey between a large or medium sized town and a small village. If I lived in East Grinstead I would call Chelwood Gate a local village.
This may just be a matter of personal distinction, as (obviously) I wouldn't - perhaps the degree or urban/rural defines what you consider local? Google Maps tells me 8 miles from Manchester gets me well beyond Stockport (Stepping Hill Hospital, in fact)... I definitely wouldn't consider that "local"!
Plusbus isn't really the same scheme in different areas of the country. In the former PTE areas it usually covers an entire county. In some small and medium sized towns and small cities it covers an arbitrary short distance from the city, but sometimes it also includes adjacent towns just because e.g. York Plusbus goes to Stamford Bridge. In the Peak District, Plusbus covers journeys of more than thirty miles from Matlock or Buxton. And of course, in Greater London, there's no Plusbus.
As regards PTE areas, see comment below on more general ticketing. Plusbus Peak District seems to be more of the relatively genuine aim of Derbyshire County Council to promote public transport usage. However, for most of those "arbitrary short distances from the city", large or small, it appears to be defined at the edge of what could plausible constitute the town bus network. With large rural expanses between areas, Chelwood Gate can hardly be considered on the town network of either East Grinstead or Haywards Heath, nor can Wimborne be considered part of 'Greater Bournemouth' when it's in a different council area!
[referring to Folkestone] I think that this is the point. The areas have seen at least some attempt to make them actually useful.
The Folkestone PlusBus area conveniently coincides with the Stagecoach Folkestone dayrider area. There may well be lots of benefits, including ease of staff training, to have your area products match.
The purpose is more to encourage people to travel sustainably to the railway, and not default to the far more damaging methods of car or taxi. This is the reason why plusbike was also tried.
I agree - I don't want to suggest that PlusBus is a bad idea (provided the bus drivers don't give you the 'can't use a train ticket on a bus' speech), but I think the real concept of the scheme has to be borne in mind... it simply wasn't designed to allow bus travel to or from wherever you like into the rail network, or, as the OP admits to doing, discarding your rail ticket and using it as a cut price bus ticket.
[...] The bus provision at Alnwick station could be far better, and bus travel should ideally be free for rail ticket holders to Alnmouth, or there could be an additional destination at say a 50p premium each way. The bus companies reasonable costs could then be covered by the railway industry. In exchange the railway would get more passengers.
I noted one of the other posters upthread said the rail company gets their full fare, the bus company the add-on only. Assuming that is true (and with no reason not to), your suggestion here would require the railway to forfeit some of "its" portion for a more balanced distribution of costs - and the disinclination to do that may well be the reason why such doesn't exist, or why doing it here may then result in demands for the reallocation of funds in other schemes too. Presumably the DfT, as current purse string holder, would also have to agree with that.