Pateras, I should think you are right about it being cost.
I just can't see the data gathered via the app being as accurate as that from antibody testing. My understanding of the app is that it is very dependent on people putting the correct information into their smartphones about whether they have or have recently had symptoms of Covid 19? Surely the majority of those inputting this information will be making an assumption of having or having had the virus, rather than having been tested properly and knowing for sure.
Also, whilst many of us do have smartphones, a significant number still don't.
I think testing properly is a much better way forward, but is the government now beginning to panic about it's finances and the damage to its magic money tree, so is trying to wriggle out of paying for tests? I think the app is potentially a false economy.