I am not aware of any data source that provides information on rural and non-rural petrol prices. There is anecdotal evidence that petrol prices in rural areas are likely to be higher than non-rural areas.
There are a variety of reasons why this is likely to be the case. These include:
Fuel throughput, in that the higher the throughput the lower the retail margin required to give a return on the investment. Rural garages are likely to have lower throughputs.
Bulk delivery charges, many sites have relatively small storage tanks since they are cheaper to install. However, they require more frequent replenishment and this results in some sites paying higher delivery charges. Again smaller tanks are likely to be more prevalent is smaller, rural petrol stations.
Competition; large, particularly supermarket sites in close proximity to one another in urban areas do tend to lead to lower prices.
Regional petrol prices are available on the AA website at http://www.theaa.com/motoring_advice/fuel/index.html.