I assume the ditched UK app is based on similar technology to the COVIDSafe app in Australia, which in turn was based on the Singapore TraceTogether app, etc. According to recent reports https://www.abc.net.au/news/science/2020-06-17/covidsafe-contact-tracing-app-test-documents-rated-poor-iphone/12359250 , the effectiveness of the Australian app on iPhones has been improving over several recent updates - although it still seems to have been of no practical use in tracing any actual community-transmission contacts to date.
(Note that community transmission is very low in Australia at present, so this may not be a meaningful test of effectiveness - and there is no Google / Apple app in Australia, so there is nothing to compare the effectiveness of the two approaches.)