"So out of interest what happens if two people drive and you aren’t sure who drove that day. Do they just fine the owner of the car?"
The registered keeper could be prosecuted for failing to identify the driver. The relevant law states
A person shall not be guilty of an offence by virtue of paragraph (a) of subsection (2) above if he shows that he did not know and could not with reasonable diligence have ascertained who the driver of the vehicle was.
This is a 'reverse onus' sort of thing - the defendant has to prove that they don't know who was driving, rather than the prosecution having to prove that the defendant does know.
If you get a Section 172 notice and don't remember who was driving, you need to try and find out. Ask if the police have a front photograph, look through old texts and emails, contact possible drivers, etc. And have a record of what you did, in case you need to tell the court that.
You should not guess, and definitely should not name someone falsely. Giving a list of 'possible' drivers isn't legally sufficient either, you are expected to name the actual driver.
But OP, if you deal with it pronto, I think there's a good chance the courts will accept 'resetting' the timescale, they won't take any action about the address problem or the late reply by you, and just send you a regular fixed penalty notice for speeding.