Really? I haven't seen any evidence of the alleged offence, how can I just be assumed to be guilty without me even seeing any evidence, that's not how the justice system works in the UK?
That is the system. Don't be late in your reply or it gives you worse issues.
The police can prove postage (almost certainly). Delivery is presumed because the law says so. It's not great, but you can attempt rebuttal. It has been rarely known to succeed. Take all your evidence to court and give it a whirl. The consequence of failure has been pointed up.
If it were the case that "I didn't get it" provided acceptance I suspect there might be a substantial increase in the number of not guilty pleas.
Personally, given RMs own statistics of 14/10,000 things going missing from their own test, I think this does mount up to fair number of people who do have a valid defence if only they could persuade a bench.
At the moment there is a request for driver details. Typically the next step will see an offereing of disposal for £100 and 3pts. But it could be a course or a prosecution.
The system does that in order to allow those who readily accept their guilt to deal with it in a simple cost effective manner. It doesn't offer "well if you can prove it I will accept the easy option".
You have no entitlement to evidence. If you wish you can elect to go through the process of court. There is a high probability that they will show a photo of the vehicle commiting the alleged offence as described. But a COFP or course is off the table.
A common practice is to ask - without using "evidence" for "photos to assist identifying the driver". They often oblige. But it is in their gift.
Another issue is that you commit a more serious offence if you don't answer in time, the clock is still ticking. Also in cases where there have been delays they may not make a fixed penalty offer because it would allow a timeout to be engineered.
You have very limited time left. Some forces will put pictures on line. It could be footage from operation snap.