Sorry,i don`t own a smart phone and have absolutely no interest in having one ever.
It`s not "Law" at all,the gov is "Urging"
see here,
https://www.gov.uk/government/news/businesses-urged-to-prepare-for-nhs-covid-19-app
they are also very aware of this,note the date, (As of 19 March 2020, COVID-19 is no longer considered to be a high consequence infectious disease (HCID) in the UK.)
from here,
https://www.gov.uk/guidance/high-consequence-infectious-diseases-hcid#status-of-covid-19