Always rather curious about these places; what was the original or intended criteria for somewhere to be a Parkway station - did there have to be a certain number of parking spaces for example?
There was no criteria as such and the definition of Parkway is not a precise one at all. Any station offering inter-urban services can attract travellers from quite remote origins.
Parkway stations came about in the 1980s with the objective back then being "to increase car accessibility to the rail network" (1). They were there to act as a convenient out of town station for inter-urban railway journeys. (2)
There was 3 different types (2, Page 126):
1) A new station close to suburban population located in a strategic position on the road network to complement existing urban stations where car access is impeded by road congestion, such as Bristol Parkway.
2)A new station to serve remote population off the main line, such as Tiverton and Bodmin Parkways.
3)The expansion of car parking at an existing station where road access is good and the station can be re-marketed as a Parkway, such as Didcot.
1- Transmark (1988) West Yorkshire Parkway. Prepared for InterCity Eastern Region, British Railways Board
2- Lythgoe, W. ; Wardman, M (2004) Modelling passenger demand for parkway rail stations. Transportation, 2004, Vol.31(2), pp.125-151