Age limits to rent cars in the UK

It depends on the agency. It can be 70,75. An IDP is not needed since those area speak English.

We booked recently via AutoEurope in England without restriction ... and I'm 75.
That said, your best option might be to do a Google search to find a definitive source rather than relying on the anecdotal evidence you'll find here, especially since the rules can change rather quickly these days. It's what I plan to do myself before our trip to England in June. .

Edited to add: here are a couple of blurbs regarding the subject from the AutoEurope website:

"The existence of a maximum age limits is a rather recent thing, which is becoming more common among car rental companies. The idea of the restriction is to reduce the risks of older drivers in traffic, and rental companies want to avoid any potential major traffic incidents. The average maximum age is 75 years, but car rental suppliers can choose to set their own limit. The good news for senior drivers is that this restriction does not exist in most European countries. As with young drivers, surcharges can also be charged to senior drivers. In addition, many suppliers require specific documents from older drivers of a certain age. These include, for example, a doctor's certificate of good driving health, a letter from an insurance company that explains any recent accidents and information on the validity of a driving license. For senior drivers, car rental companies can also limit the availability of certain cars and car classes. Maximum age limits may only apply to some car classes. Additional drivers are also subject to the same age limits as senior drivers."

and this:
"When you make a reservation with Auto Europe, always inform us of the right age of the driver so that we can notify you of any age limitation for the car hire. We confirm whether you can drive your chosen car depending on your age and we will let you know of the possible fees on your booking confirmation. For more information on fees and other possible limitations, please see the terms of your booking or look them up on our website. If your local rental company requires some documents or additional information, we will let you know. If renting a car of your choice is not allowed, we will notify you personally. Our qualified customer service staff will help find you the right deal for your travel plans, regardless of your age. If you have any questions or need more information, please contact us by phone or email seven days a week."

We ran into trouble on Orkney. My dad booked it and he was 80 I think. I ended up doing all the driving. The young girl was so funny, "but you didn't sound 80 at all in your emai!" Hah, hah.