I'm writing this after flicking through TV Crunch's
25 Worst Sitcoms Ever. Although it's US-based, the 'winner' was the not-fondly remembered
Small Wonder. They ran this on ITV in the late 80s I think and I remember at the time, even as my less-discerning younger self, how indescribably shit it was.
(Annoyingly, the site also lists at #13 a sitcom starring George Foreman called
George, which UK viewers were sadly never given the chance to see.)
The other one that sprang to mind was
All About Me, which starred Jasper Carrot and Meera Syal. Presumably trying to tackle issues such as disability, mixed marriages and step-children, the writers completely forgot to write any jokes. Simply jaw-droppingly bad.