Outdoors GPS with Parks + a £12/yer subscription for 1:25 and 1:50 OP Maps. Looks like they now only have “Outdoors GPS – Offline OS Maps” for £1.99 so no included national parks maps.
Allows plotting of routes and import/export of GPX on the desktop, plus phone app although its a bit (lot) harder to plot on the phone. I find switching between OS maps and Google satellite and google maps to be the best combo for route planning on and off road.
It isn’t so good for actually following the route on a bike though as the map doesn’t move with you as you move, or orientate itself to your orientation.
Koomot is better for actually following the route and better for battery if you get offline downloads when route following too. Its like £30 for the entire world downloaded at the moment.