In reply to jockster:
Have a look at OSMand.
It is about £10 a year.
It is based on open source mapping, which I find adequate, but not as good as OS maps, especially when in more open terrain.
However, it is a relatively small file size to download all of the UK. Therefore it loads quickly and is available without a 4G signal.
It also covers Europe.
It is a bit clunky to use, but it will accept a gpx file and provide 'turn by turn' instruction.
There is a useful community of users: https://www.facebook.com/groups/826236498324403
https://play.google.com/store/apps/details?id=net.osmand
Cheers,
Ian