I have rented from the Hertz London Marble Arch office, but it was before Covid. I have also rented from Hertz and other companies all over Europe. I have never been asked to show an IDP. The IDP is simply a certified restatement of the important information on your DL in multiple languages. Some countries "technically" require it, but I have never been asked for one.
When they are referencing non-EU drivers that includes licences from multiple countries where English may not be the language, so there may be an issue in understanding what the licence says. US and Canadian licences are in English so that is not an issue. On the other hand, if you are in a country that requires one, make sure you are not in an accident or pulled over by the police, since you may be subject to a fine. For the small price of an IDP, if it isn't overly inconvenent, why not just get one? It could save a potentially major hassle. I usually do when in Europe.