Two pages is the standard answer for most UK professionals - but the real rule is that length should follow relevance. Here's how to judge it for your situation.
Most professionals: two pages
Enough room to evidence your experience without padding. If you're filling a third page, you're probably including detail a reader doesn't need.
Early career or graduate: one page
With limited history, one focused page usually reads stronger than a stretched two.
Senior and executive: two to three
Leadership roles can justify a little more to show scope, strategy and outcomes - but concise still wins.
Cut by relevance, not by age
Older or less relevant roles can shrink to a line or two. Recent, relevant experience earns the space.
Make every line earn its place
If a bullet doesn't add evidence, scope or relevance, remove it. A tight two pages beats a baggy one.