After several experiences, I’ll say International Driver’s Licenses are (mostly) useless documents. I’ve gotten them for travel in New Zealand, South Africa, Morocco, Fiji and more.
First, getting it at the post office illustrates how useless it is. It is not tied to your driving experience (eg. No test or education on foreign driving).
Secondly, no one needs to see it or even knows what it is.
Having said all that, I can see the potential and will continue getting them. The document looks official (with your photo and a stamp) and explains, in every language, that you are licensed and able to drive in that country.
That could be invaluable if you are involved in a collision where you don’t speak the language. The common person will demand to see identification and will only be familiar with their own license card.
Now... this being the UK Forum, I assume English is your first language. So, you will be fine in the USA. Everyone that matters (police or car rentals) will understand foreign licenses. If something else happens (collision) you can talk to the other party in English.
In regards to the EU, I have no experience but I can’t believe Brexit will suddenly make a UK Drivers License an unrecognizable document. You are still legally allowed to drive in any country in the 1968 Vienna Convention (see attached link for a complete list of countries).
“An IDP is not required if the driver's domestic licence meets the requirements of the 1968 convention; the domestic licence can be used directly in a foreign jurisdiction that is a party to that convention.”
https://en.m.wikipedia.org/wiki/International_Driving_Permit
