I have had no problems with the NHS app nor the Guernsey tracker.
If you are being requested a photo then I don't think you are using the NHS app, it does not request that, you do need your NHS number. If you don't know it your local surgery should have it.
The NHS app is quite handy, I was already using it, I can access my medical records, vaccinations going back a very long time, results of tests eg blood and medicines being taken. It took a few weeks for all that to appear as Southampton hospital have to enter the details, for those in Dorset it will be Poole/Bournemouth hospital. To the lucky ones who have absolutely no medical issues at all it would be less useful!