When to get tested?

[WouldShouldCould]

I've posted on a thread but it's v fast moving so not many replies.

Neighbour has a temp, sore throat and a cough, my kid appears to be going down with the same thing. Neighbour called 111 but hasn't been sent for tests as not been abroad/hasn't been in contact with any known case as none in my county (yet, agreed that it's probably here already but no reports)
Does it matter if they don't test? Not a high risk patient and it's probably a big standard virus, just wondered how anyone would know the true numbers if people don't get tested.

All the advice I've heard is just to self isolate if you think you may have it. And if you have friends and family to help by dropping groceries or meals off to the front door. That being said it is just as likely to be a normal cough or cold.