I suspect that may have been more out of ignorance or lack of awareness, that it didn't occur to someone that in other countries there are equivalent alternatives to having a solicitor "certify" a copy, and once someone has laid down a rule, it's the easiest thing to tell everyone that they have to comply with the rule.

A job I had for several years in the mid 1990's in London involved a lot of certified copy of passports and of other documents, and they were coming from around the world - from Far East to West Coast, as far south as the antipodes, and many places in Africa and we accepted all certifications by lawyers and notaries, per advice from the solicitors advising my employer. Of course in the pre-internet days, the funny thing was that we had no practical means of actually verifying any of the certifying authorities, and having an embossed stamp available was apparently the greatest hurdle to overcome to send us any ol' garbage passed off as a certified copy.

I know. That said, I used to hold a California notary commission and we were taught that certifying a copy of a document was NOT a valid notarial act. Nonetheless, there is a common misconception that California notaries can perform such an act.

Well what they do is good enough at least for some purposes- when I needed a certified copy (I can't remember what for) I was asked to write that it was a copy and sign, and the notary verified just my signature, and it was accepted in the UK.

A certified copy of your non-British passport is never required nor requested. HMPO asks for a colour photocopy of every page of your non-British passport and they reserve the right to request the original. Practically speaking however, if you choose not to send a photocopy, you will still receive your new British passport. You’re actually more likely to have your application refused by sending such a copy than not.

Ditto NY. I recently was asked to provide, to a UK financial institution, a certified copy of my passport and driving license. I then discovered that notaries in NY will not do this because it is not considered a valid notarial act.

Yes, as per post above, they will not notaries official documents. But the same notary, a couple of weeks later, was happy to notarize my signature on a form for the providers a UK-based occupational pension which, of course, is not considered an official document.

Question on this for my own curiosity: were you going to mail your passport renewal during a trip to the UK? But you have to send in the passport you're renewing don't you (your "oid" UK passport)?

Why do you say that?

Yes, but I'm a dual citizen (which was the whole point of this thread), so I was traveling on my US passport.

I have had copies of identity and address documents notarized and accepted by several UK financial institutions, none of them had any issues with that.

I thought you had to travel in and out of the UK on your British passport?

Why would you think that?

That has never been the case.

You can travel in and out on whichever passport you prefer.

I've kept the rule of using whichever passport of the country I'm in to travel in and out of. I can't find any official guidance on it though so maybe it is okay.

Might be confusing it with the requirement to travel in and out of the US on a US passport if you are a dual citizen. The US won’t recognize your British citizenship if you are a dual citizen.