I use Viewranger on my phone... you can download OS maps as you need them and it works offline so doesn't need a phone signal.
But tbh I just use it as a back up as I'd rather leave the phone battery for when I need to make a phone call
After trying to use the Garmin 500 etc and getting lost because it loses gps signal under tree, bread crumb trails are hopeless etc I decided to get something better mainly because I started riding with my family and a 10 mile detour would be frowned upon.
Looked at the cycling ones but various online reviews and threads about them being a bit lacking on occasions I decided to forget the cycling part and just buy a good gps nav,
I went for one of these
https://buy.garmin.com/en-US/US/into-sports/hiking/oregon-650/prod113520.htmlIt's a big ugly brick that's hard to mount on the bike ( managed it though
)
But....
It's 'rugged' and waterproof to IPX7 ( you can drop it in a stream ). It uses AA batteries so carry some spares and just change them if you need to.... mine last about 4 - 5 hrs with the screen on or ages with the screen set to auto off.
Most important thing though... uses both the American and Russian gps satellites and will get a signal lock while I'm sitting in my office with no windows through a internal ceiling and my industrial units roof..... a good enough lock for it to track me walking around inside
You can load OS maps but I use open cycle maps.
TLDR....
Bad... expensive, ugly not really for bikes.
Good... proper not messing about gps that works when you need it to.