The Register has a very technically savvy long-read headlined simply: “UK finds itself almost alone with centralized virus contact-tracing app that probably won't work well, asks for your location, may be illegal” theregister.co.uk/AMP/2020/05/05…
Basically they highlight the way iPhones will “work” when locked in the NHS system; they, uh, won’t, unless they are near either an Android or someone with their iPhone which is unlocked, screen on, app foregrounded. 
The takeaway: Two people who have their iPhones locked in their pockets will not register as contacts with each other. A room of people with iPhones locked in their pockets will not register with each other unless someone with an Android is in the room.
This is effectively a form of Android Herd Immunity: for the good of Britain, vaccinate your friends by giving them Androids!
The Register has updated its article to say the situation might even be more complex, with Android relying on foreground services. Apple devices seem to also require users to manually relaunch the app. order-order.com/2020/05/06/tri…
