I have two Kindle Fires - and the official Ordnance Survey map app - Mapfinder - won't work with either because Kindles run an old version of Android. Can I side-load the latest non-Kindle version of Android, so apps work - from the Kindle App Store - and still keep the front end, which has nothing to do with the operating system. Failing that, can I just install the latest version of Android, as used on the latest Android devices, and still keep access to all my Kindle apps and capabilities. I don't see why I should buy another Kindle every year or two just to be able to run apps from the store - and why don't they tell people that many apps in the Kindle store will NOT work with a Knidle - even the most incompetent programmmer knows how to code an installer to check the version of OS and tell the user the app isn't compatible. FFS, even APPLE manage that on their iOS and OS/X devices. Get your fingers out, Kindle, and either update all Kindles (no matter what generation) to the latest version of Android, or else fix the store so it doesn't offer incompatible apps - Ordnance Survey says you should be able to run the app if Android is up to date - but not if you own a device like a Kindle, that happily installs the app, then claims it can'ty see the internet, when it's actually connected and working.
I don't see why I should buy another Kindle every year or two just to be able to run apps from the store -
You appear to be talking about a Fire tablet, not a Kindle. The OS simply isn't upgraded often enough on the Fire to require you to "buy another Kindle every year or two". The latest ones (the HD 8 released in 2018 and the new Fire 7 released this month) are on Fire OS 6 (based on Android 7.1 Nougat), while everything else back to the tablets released in 2014 (5 years ago) run Fire OS 5.
The only way to get a "standard" version of Android on a Fire tablet to root it, which may or may not be possible, and is information you would have to seek out elsewhere. Frankly, you would no doubt be better served in this instance by giving up on trying to utilize the Fire and buying a tablet (or even just a large screen low end unlocked phone) that meets the necessary requirements.
In the case of the app you are complaining about, the developer has made it clear that their current app (OS Maps) is not compatible with the Fire, that only the old no-longer supported app (OS Mapfinder) is compatible with a Fire tablet. That app (OS Mapfinder for Fire) has not been updated in 5 years and as such does not need the latest version of Android. (even the regular Android version has not been updated in 4 years) The minimum spec for Mapfinder is Android 4.0. They also clearly state "If you cannot find the app in the Kindle app store it is not compatible with your device".. which means, when you don't see it in the app store on your Fire then it is not compatible with your Fire. When you side-load an incompatible app like that (one you can't get onto your device through the store) you take full responsibility when it doesn't' work.
In the case of your general complaints....
First.. the Amazon App store will not allow an incompatible app to be downloaded to a Fire, and the product page does in fact tell you what devices it will run on. When you are viewing the app store on a Fire it won't even show you incompatible apps. Not in the store and not in your library of purchased apps. When you view the app store on a PC and you are logged in, it will show you which of your registered devices it is compatible with.
Second... mobile apps do not have "installers" in the traditional sense. Installing is handled at the OS level. The installation process does not perform an OS check. That is not the way mobile apps and operating systems work. It is designed such that the app stores are expected to block devices from downloading incompatible apps. If you are somehow able to get the app sideloaded onto a device, both Android and iOS will happily "install" the app regardless of if they will work or not. Sideloading apps is always done at your own risk.
NOTE: I do NOT speak for or represent Amazon in any fashion. This is a customer to customer forum.
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"Ordnance Survey is the national mapping agency for Great Britain. Since 1 April 2015 Ordnance Survey has operated as Ordnance Survey Ltd, a government-owned company, 100% in public ownership. The Ordnance Survey Board remains accountable to the Secretary of State for Business, Energy and Industrial Strategy"
it wouldn't be offered on amazon.com in the first place, just on amazon.co.uk
Problem is, I'm thinking it does show up as compatible - even for the newer Fires...
Yea, you are right. It is shows up as compatible with all models except the original Fire. That is odd. Another oddity is that there seems to be something else wrong. If you go through the reviews, starting at the end of March the app seems to have suddenly stopped working. Before the end of March all the reviewers could at least use it. A few didn't like it but they could download and use it. After the end of march almost all reviewers can't use it. For some it only downloads to around 40% and others end up with the "No WiFi" error the OP mentioned. Only a couple mentioned being able to use it. It might be time for Amazon to pull it.