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Lezyne also makes some cycling computers with simple mapping, they start at sub-£100. "Lezyne Mini GPS Cycle Computer" is £80, a friend of mine has it and seems to work for him. I won't vouch for non Garmin/Wahoo products for nav/map but if budget is a concern you can give it a try and see if it performs up to spec. It shows just a trail to follow and your position just like the Garmin Edge 25 in the above reply. Tiny tiny fonts though.
Do you want airplane mode though, don't you want GPS on? Otherwise it's fine seeing the route but you might not know you're on it...