I recently downloaded the Android National Trust app and was disappointed in it in several respects.
It was difficult to use the map to find somewhere to visit, requiring several zoom operations before you can select a place. Unfortunately, once you have got that far, the app crashes.
This lead me to thinking of writing my own app but, as The National Trust is a charity, they are not required to publish data and do not do so. Whilst I am in the process of extracting the data from Wikidata, I thought I would have a look at Listed Buildings. There are a lot of them.