If the police want to know who you are in the UK they ask for details such as name, address, driver's licence and so on. We have no history of having our DNA tested to prove who we are. There is no reason why we should be expected to have a DNA test to prove who we are not.
I don't think the police would ask for a DNA test unless normal identity checking methods were inconclusive. For most innocent parents normal methods would quickly demonstrate who a child was.
The reasons being given for submitting to a test are illogical, and none of them over-ride our right to be left alone to live our lives in a free country. The UK isn't a police state and we have the right to be respected and presumed innocent unless there is evidence to the contrary.
I expect some would see it as a good idea for all babies to have their DNA placed on a database for life. That would make police work easier in a lot of cases, but it would be strongly opposed. Issuing identity cards to everyone was strongly opposed. You may like to try and work out why that is.