UK Government Contact Tracing App safe?

The UK Governmtn’s National Security Cyber Centre recently released this paper on the planned contact tracing App for Covid-19.

First of all, I would like to say that I know that a contact-tracing app endorsed by the UK’s spy agency is certainly not private, at all.
Nevertheless, the paper goes into a fair amount of detail on the technicalities of the App but as I am not very well versed netsec i was hoping that someone might be able to point out if there are any glaring issues with the application as stated in the paper.
Also, I intuitvely understand why a decentralised system would be better for privacy. However, I don’t have the technological knowledge to understand why that is the case, or refute what is set out in the aforemnetioned paper as the problems of a decentralised system. I was hoping someone might be able to explain that to me since i know most european countries have gone with a decentralised system instead but I don’t understand why.

Thanks in advance!

well, just biggest problem right now that you need to test corona to get the app which someone like me wont test for corona and as they said Bluetooth won’t work on most of phones and finally all data will be in one place so one little hack and all data will be online for anybody also how not personal ? if they literally can get your phone ID ? so yes person beside you won’t know but company will know also last little tiny thing HOW I WILL TEST CORONA AND BE OUTSIDE ? IF I GOT CORONA THEY WILL MAKE ME STAY IN HOSPITAL SO WTF ? I don’t see the full picture or whole thing is really messy so i need explain

It’s more about figuring out who you’ve been in contact with etc. People can catch the virus from you, but not show apparent symptoms (for quite some time or at all).

Personally if I mixed with a lot of people I’d have a old phone with just that app on it and nothing else. When we’ve got a vaccination and the virus has been handled I’d just wipe that device.

It doesn’t appear to need internet access, so it should work without a SIM card.

it needs internet access because it will call to it’s servers and yes you will wipe your phone but app won’t just disappear maybe they will still using it and just create new way to surveillance us

Likely though it just synchronizes when you have network access, so that could be WiFi or anything, it means you don’t need a second SIM card.

The reason for having a separate phone is because then the only thing you’re sharing is bluetooth, and device ids that you don’t really care about.