PokemonGo uses Google Maps and overlays it with fancy graphics (how else do you think they're getting accurate maps of your area?) So every time you play PokemonGo you're using Google Maps. That's a lot of data and a lot of battery life.
On my android phone you download it this way.
1) Go to Google Maps, search for the area you want to download, i.e. Bradford, then click on the 'hamburger' menu icon in the top left corner.
2) Choose 'Offline areas' in the menu:
3) Click 'Custom area'
4) Click 'DOWNLOAD'
Done From now on in this area PokemonGo will be using offline version of your Google Maps.