[BasedInfra]

You can put just ‘HMRC’ (UK tax department) on a letter (with paid postage) and it will get to them.

[ClapperHeid]

...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?

[YurgenJurgensen]

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.

[billpg]

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.

[Y_Y]

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.

[ClapperHeid]

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.

[agreement5051]

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...

[voisin]

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

[chipsa]

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

[xioxox]

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".

[dboreham]

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

[Symbiote]

"Now" was 33 years ago!