I really can't see any issue with downloading and using it.
The app is, by and large, going to be a complete waste of time.
1. A large percentage of the population will have older smartphones, with OS versions not compatible with the app.
Most will not relish the idea of inflicting a higher monthly cost on themselves to upgrade their phone just so they can use a T&T app with unproven benefits.
2. Amongst those who have the app downloaded on their phone, there are many, many reasons why they cannot have their phone by their side at all times. It is far too simplistic to say 'well switch it off if it's in a locker'. I can think of several reasons why someone, eg a temporary agency worker, would not wish to comply with that.
3. Bluetooth on and active will drain a smartphone's battery even more than normal. There are Google; and likely similarly Apple; apps running now in the background at all times that drain battery power as it is. Without bluetooth making the situation worse.
The govts T&T app development team probably ran with the expectation that everyone had the newest smartphones and they would be switched on and inexhaustible battery life for at least 14 hours of a 24 hour day.
The T&T is only working well in countries that have a good manual test and trace system with actual people to follow up.
In this respect Scotland is doing reasonably well. Got its act together over the summer whilst cases and deaths were low.
England, meanwhile, did fuck all with more blather and bluster 'moonshot' buzzwords, meanwhile
zero meaningful action.