I made the questionable choice to write my map renderer from scratch, got it barely working, then got bored and abandoned the project at least for now. When I finish my current pet project I plan to replace it with an existing renderer and focus on the hiking/backpacking specific features I can add.

If you want something that actually works I highly recommend Gaia. They only have OS's driving maps built-in, but you should be able to manually add OS's leisure maps. Reasonable usage will definitely fit within OS's free tier, but you'll probably need to pay Gaia for pro. Disclaimer: I've worked with the tech this described a lot, but not actually done the steps I'm suggesting.

See:

https://osdatahub.os.uk/docs/wmts/overview

https://help.gaiagps.com/hc/en-us/articles/360047068634-How-...

My WIP project: https://github.com/danielzfranklin/backpackingmap