Letters from Public Representatives

They won't be using the registered owner. They may be using the electoral register (they should not be).

Did you ever contact that rep yourself directly?

In a world where I can legally ask a company to stop sending me unwanted emails I think it makes sense to allow people to opt out from receiving unwanted letters from politicians.

.....

One with significant restrictions on use. It cannot be used for marketing for instance.

A letter advising of a new supermarket could be claimed to be important under that basis also. That absolutely isn't allowed.

"Electoral purposes" is explicitly allowed (and even has no right to object to it, uniquely - you can't demand someone stop). I cannot find if this requires it to be within an election period or even if that has a definition outside of the spending period - which has not started for the local elections yet and likely won't til April.