https://help.cbp.gov/app/answers/det...riminal-record
Now that didn't take more than 5 seconds to find on an official US government website.
In practise it is all down to individual officers discretion.
You must declare it if it is on your record. The US/UK share this information so they will be able to see it.
Lying means you will be denied, and that then counts as fraud under Moral Turpitude which definitely means you will be denied entry at future attempts.