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I'm looking to buy a second hand mountain biking GPS unit. This will be for Sustrans route, The Badger Divide typr rides etc... ideally £100 to £150. I just need a basic but very easy to follow screen, not interested in miles or climbing etc.
If you can get a Wahoo elemnt bolt, I'd suggest that.
I love mine. Really simple. In the age of the smartphone, it can be frustrating when you keep trying to swipe or pinch and zoom and realise you can't, but that never works in the rain on touchscreen ime anyway.

Yeah, Bolt is probably the best choice for that if you don't anticipate deviating from your planned route. If you want cheap as chips and don't need any mapping at all, then an Edge 500 might be worth a look, too.
that never works in the rain on touchscreen ime anyway
Works fine on the new Garmins, but that may be overkill for the OP’s needs. Might get a used Edge Explore for £150 though, and even a new one’s only a bit more.
Though there are extremely mixed reviews of the 820. I tried one long ago in a shop and the touchscreen was borderline unusable; enough for me to never consider one. Garmin's newer efforts are much improved on that front; I've no idea whether newer 820s have better screens.
Also if the requirement is "easy to follow" on very specific routes then the clarity of the Wahoo's display, especially in sunlight, is much better than any Garmin (though the 130 comes close, IIRC). Where the Wahoo starts to fall down is where you want more than just basic navigation of a pre-planned route.
We have two old Android phones running Viewranger, Google maps, trail forks and Strava.
We pre-download before we go.
It's not turn by turn unless it's Google maps, but it works for us. The one time I needed them in front I elastic banded one to my stem.
Both on pay as you go, costs maybe £10 a year with occasional data use.
They are waterproof Moto G3's in bash proof case, about £20 each or I've three in the cupboard at work...
Im as matt ^^^ I use my old OnePlus One. I buy the physical map to get the digital download code (means i have the os map downloaded offline to all my devices with os app) then just piece routes together on the os app. Use a Decathlon 900xl phone holder mounted to stem. Airplane mode on, the old OnePlus did 5 hours with screen on yesterday... very pleased with that given the minimal cost and getting to view os 25k maps on a decent size colour screen
With a phone, you are draining the battery with constant use, especially screen on (and a bit with an app actively using GPS). Noting you may need the phone in an emergency.
Personally I'd use a dedicated GPS to free up the phone for photos, the odd detailed map check on ViewRanger etc, and emergency calls (even then a day of doing photos and a few map checks can drain my phone which normally has a day and a half battery life if sat doing nothing. GPS as well would kill it in half the time).
Other option though is a USB powerbank attached to the phone. Considered that with a phone bag for the bike that I've got, but there's not enough space to fit the USB cable into the phone and powerbank. Maybe an option if you stop for lunch for half an hour or more and charge the phone.
Garmin Etrex with OSM mapping (Talkytoaster).
Noting you may need the phone in an emergency.
That's why I have old navigation phone and my shiny new phone phone...
That’s why I have old navigation phone and my shiny new phone phone…
Is one for going ‘out’ ad the other for going ‘out-out’??
‘out-out’
