James Vincent, The Verge:

A beta app launched by the UK this week shows the huge challenges they face and, crucially, the difficulty in designing an effective app without the help of the tech giants that make our phones.

And:

Instead of decentralizing the data across devices, the UK will pool the information it collects in a single database operated by the National Health Service.

And:

In addition to privacy issues, researchers have identified a major problem in the UK’s efforts to build an app without Google and Apple: it simply won’t work as advertised.

One major problem is Bluetooth pinging. From this Register article:

Apple’s iOS normally forbids applications from broadcasting via Bluetooth when running in the background. That means you would have to leave a contact-tracing app open in the foreground all the time for it to work properly.

Read both articles for the details but, in a nutshell, the centralized data pooling approach seems to rely on people leaving their app running in the foreground (rendering the phone useless for all other purposes, not going to happen) or negotiating exceptions with Apple to allow Bluetooth to ping in the background.