It Is Encoded In Our Legal System, Like It Or Not
Is there any way to declare "I don't want physical post" in such a way
that anyone "taking reasonable steps" to contact me can't count
physical post as "reasonable steps"? I don't see a declaration of "I
live here" which is required by law in certain circumstances equates
to "sending letters here will reach me" ... and it is an assumption
made incorrectly and encoded in our current legal system
I think that you've figured out the problem. This is encoded in our current legal system. There are all sorts of laws, case law precedents and contract terms that hold as a matter of law that a letter sent through the postal system addressed to your physical residential address is a reasonable way to give notice to you.
Problems With Designating A Non-Residential Address As Your Residence
This puts you in a bit of a bind.
On one hand, if you use the address where you usually do receive packages as your residential address, but you don't really reside there, you are being untruthful which could be a problem in and of itself, and it could have all sorts of unintended consequences (e.g. it could cause you office to be charged with violating land use laws that don't allow that building to be used as a residence, and it might cause you to pay commercial rather than residential rates for utility services at home).
You could also be accused of evading service of process via a process server if a process server shows up at your purported residential address and finds that it is not, in fact, a residence, which could cause you problems. Indeed, as a lawyer, I've encountered several cases where defendants who are con-men or otherwise engaged in fraudulent activities declare a residential address that is really a private mail drop even though the actually live in a house rather far from that location for the express purpose of making them harder to find or to serve with legal process, and have had to seek extraordinary relief from the courts to obtain substituted service of process on these defendants as a result.
On the other hand, I don't think that there is a way to opt out of receiving physical post as an individual.
Collective Abandonment Of Physical Addresses
I am familiar with some entire communities that manage that, however.
For example, at many colleges and universities, students can take mail exclusively at a post office box in a college mail room for the duration of their stay at college that is not attached to the physical address of their residence, without being viewed as deceptive or improper in any way.
Similarly, in a number of seasonal resort communities, mail is delivered only to a central mailroom and there is no mail delivery to physical addresses in the community, saving the postal service effort and providing greater convenience for seasonal residents.
General Delivery Addresses
In the United States we also have a somewhat outdated concept called general delivery mail service, although I don't know if it exists in the U.K. This is useful mostly for mail intended for homeless people or urban nomads who have no fixed physical address. Instead, mail for these people is sent to the general delivery bin at a post office that is convenient to them and they can walk into the post office to ask if there is any mail for them in the general delivery bin from time to time without having to pay for a post office box of their own.
General delivery mail can sometimes be used in a manner similar to that of sending mail to the physical address of someone who usually doesn't take mail at a physical address to give them notice, which is a bad thing because most general delivery mail is never picked up.
From a practical perspective, probably the most viable option is to claim as your legal residence the physical home of a family member or close friend where you now and then stay overnight, and with whom you are in touch by phone, etc., and then to not inform the world or bureaucratic paperwork about where you actually rest your head most nights.
This is an approach that is often used by people with careers or the equivalent that leave them on the road most of the time (e.g. explorers, archaeologists, traveling salespeople, people in maritime occupations, circus people, retirees who travel often, people who actually spend most nights with a significant other but aren't married, etc.).
As long as you have some physical address that you can call home with a straight face even if you don't spend much time there, that meets the needs of having a postal address at which you can be reached, the bureaucracies of the world will generally tolerate you, even if you don't spend much time there.