This is called the Postal rule.
Basically, provided someone can prove that they have posted a letter and provided it’s not been returned, it is deemed served 2 days after posting excluding bank holidays and weekends. If it has not been returned, the onus is on the recipient to prove that it was not delivered. That is very difficult.
In English law, the Postal Service is deemed to be 100% reliable unless the recipient can prove the opposite.
Here is an article on the Rule.
Can I clarify anything for you?
Please don’t forget to rate the service positive. It’s an important part of the process by which experts get paid.