New Travel Requirements for United Kingsom

You should travel on your UK passport when travelling to the UK

https://www.gov.uk/dual-citizenship

There isn’t an English passport.
Do you have a UK passport?

You are a UK citizen (unless you have renounced it) so you need to travel on a British passport, or obtain a Certificate of Entitlement to Right of Abode (which is attached to your foreign passport).

Your place of birth is not relevant- but your citizenship is.

How would I know whether I had renounced it (over 50 years ago) when I became an American citizen?

Generally speaking, one must proactively renounce citizenship. Simply becoming a citizen of a country does not make you a non-citizen of the other country. Here's the website for renouncing UK citizenship. https://www.gov.uk/renounce-british-nationality

As Trotter says, you don't automatically lose British citizenship by becoming a citizen of another country, you have to actively renounce it. So if you haven't done that, you would still be a British citizen.

You will need to apply for a British passport or a Certificate of Entitlement in order to travel to the UK. A passport is the cheaper option, and will be easier than you might expect.

Do you know when the requirement to have a UK passport came into effect? (Although born in London, since the early 70s, when I became a US citizen, I have traveled only with a US passport.)

It came into effect on Feb 25th of this year.
https://www.forbes.com/sites/duncanmadden/2026/01/23/new-uk-dual-citizenship-passport-rules-for-2026-explained/

This is an interesting thread. While it would probably be to the OP's advantage to get a UK passport along with their US one, I wonder whether they would actually be denied UK entry with a valid US passport and US citizenship. Would the British authorities even notice that London was the birthplace? (Don't gamble on that just to satisfy my curiosity though! ;-)

The OP is legally required to ttavel to the UK on a British passport (or using their US passport and a Certificate of Entitlement to Right of Abode). If they got as far as the border they're not going to be denied entry, because they are a British citizen. The risk is that they can be denied carriage by their airline.

Place of birth has no bearing on this - you can be born in the UK and not have ever been a British citizen, or have been a citizen at birth but renounced citizenship. Or (as many people have discovered) be born abroad and have never even visited, and yet be British.

Given the OP was born before the introduction of the digital immigration system, it's unlikely the authorities will realise rhey are British, but in law they are, and therefore are required to enter the UK using their British passport or a CoE.