Regarding the point at hand, has any justification been given for the storage of data for 20 years or has it just been plucked out as an arbitary figure? Either way, it is rather concerning
I pulled the relevant section out of their data privacy statement earlier up thread:
The personally identifiable information collected by NHS Test and Trace for people with COVID-19 symptoms is kept by Public Health England for 20 years.
The personally identifiable information collected on the contacts of people with COVID-19 but who do not have any symptoms is kept by Public Health England for 5 years.
This information needs to be kept for this long because COVID-19 is a new disease and it may be necessary to know who has been infected, or been in close contact with someone with symptoms, to help control any future outbreaks or to provide any new treatments.
Whether that is justification enough, you'll have to decide!