Out of interest, as a general rule, who is responsible for putting up info like this - GWR▸ or the local council? (and yes I know the two should be working together but I suspect that doesn't always happen as well as it could).
I assume that as the station will be owned by GWR any info that does go up would need their permission anyway?
The station operator is required to provide certain information, mostly about the station and train operation, and there's a common national format "information about the area" poster which is usually provided; I'm not sure if that's a franchise requirement on GWR, more general, or something that's generally done because it's sensible. That latter poster is highly stylised and standardised, and is a nightmare to keep up to date as it lists onward destinations and bus stops and routes.
The local council has - to my knowledge - no obligation to provide a welcome or information at stations, though in my view it's a good idea for them to do so.
Where a poster goes up on Network Rail property that's leased to a train operator as part of a franchise, some form of agreement is necessary. And the hardware, etc, necessary needs to be vandal proof to avoid it getting chucked on the line, or have any risk of getting blown there in bad weather. Content needs to be in the interest of the train operator.
Outside what can be quite narrow confines, Network Rail owns the track and surrounds, and an agreement would need to be reached with them for posters / displays on their land.
The boundary from Network Rail to leased-from-Network-Rail to council owned public highway can be an interesting one to research ... once you're off the railway land and onto the council, there's a different story. Very interesting, away from signs, where the boundary goes across the middle of a series of car parking spaces that you want remarking!
There's a lot of knowledge on Community Rail lines and on Community Rail stations as to whom to ask about what and to help bring partners together. The map at
http://www.projectmapping.co.uk/Resources/ACoRP%20summary%20map%20v3.pdf will help you identify your local Community Rail partnership if you have one.