All such schemes are dependent on widespread testing just to get started. HMG's testing target is 100k per day by the end of this month. It seems from current progress that the number actually delivered is a fraction of the claimed capacity and I expect that if the claimed capacity is 100k they'll declare the target met even if reality continues to fall well short.
However, let's assume the delivered tests actually meets that target. The current policy is that frontline staff and their households are entitled to a test. The number of households is estimated at above10 million. I wonder if anyone has worked out that even if only the staff let alone other household members are to be tested this is going to take well above 3 months. To test the existing households within a reasonable period of time the target is about an order of magnitude short. If lockdown is eased the number of qualifying households is going to increase so the testing capacity is going to have to increase further.
Now let's assume that this scheme is under way. It depends on the testing regime picking up a large proportion of existing and new infections. Without this there is inadequate data to start the system and most positive contacts will be missed. This means that the existing UK testing capacity is unlikely to be able to bootstrap the system in any useful manner.
Let's further assume that the system is up and running on an adquate footing. What happens when the positive contacts start to flood in? We must assume that a proportion, probably a majority of reports will be false positives. How should those receiving a warning react? Are they to assume the worst and go into self-isolation? What's the economic and personal impact of such unnecessary periods of self-isolation? The reports are going to have to be followed up with tests to avoid this and the testing system will have to be able to cope with this as well or TPTB will need to be prepared to switch testing strategy from frontline household members to putative contacts.
It seems likely that such a system is going to depend a testing system adequate to bootstrap it effectively and, unless the infection rate is low enough when it's introduced, a testing system adequate to not be overwhelmed by testing those with positive contact reports. It's certainly not going to be a means of easing to load on testing, nor on getting infection rates down from current levels.