...and, if you write to a member of parliament at the Houses of Parliament, you don't have to use a stamp.

So presumably...

<MP name>

HP

...with no stamp on the envelope could theoretically work.

Now, which MP has the shortest name?

No stamp required is due to “Franking privileges”. Same as in Canada.

US Congress people also have “franking privileges” but it only applies to outbound, not inbound mail.

Ed Davey, Tom Hunt, Rupa Huq, Ben Lake, Ian Levy, Alan Mak, Lia Nici, Naz Shah.

I daresay "A Mak" or "R Huq" would work too.

Since not including a stamp should imply it's for an MP maybe it would be enough to choose a unique short name. I think there's more than one Ed, but Lia or Naz should be sufficient.

Actually, thinking about it "PM" should be sufficent to address it to the Prime Minister. So...

PM

HP

Might work. Maybe I should go and parcel up some dogshit and try it.

Can we go shorter? The Director General of MI6 traditionally has the designator 'C'. This probably wouldn't work anywhere in the UK, but if you dropped it off at a Post Office near Vauxhall Cross, they'd probably figure out what you meant.

Do you have a source for that? I could only find this, which seems to say the opposite: https://www.whatdotheyknow.com/request/218301/response/54314...

I believe DVLC would work too (now DVLA, iirc).

"Now" was 33 years ago!

There are also freepost addresses, where you can write "Freepost CompanyName" with no postage. I've found one on google which is just "Freepost OAL".