uk – How long of a gap between visits to Britain is necessary to ensure entry of 90-day trips?

I was born in the US and am an American citizen who has never had a problem with my passport. I have heard that British immigration officers can be strict so I am wondering to what extent. If I travel to Britain for 90 days with 400 days between visits, they won’t cut my allowed stay to 60 days, am I right? And there should be no chance of them denying entry altogether?