Will alcohol-free beer show up on a breathalyser?

At true "alcohol-free" strength (0.05% ABV or below), no - the amount of alcohol involved is far too small to register on a standard breathalyser or to push you anywhere near the UK drink-drive limit, even after several drinks.

There is one practical exception worth knowing: breathalysers can occasionally give a false reading immediately after drinking anything, alcohol-free or not, because of residual mouth alcohol or even certain non-alcoholic ingredients lingering in your mouth. This settles within a few minutes. It is not the drink itself being detected as alcohol in your system - it is a temporary mouth-level artefact, and it clears quickly.

If you have had a "de-alcoholised" 0.5% drink rather than a true 0.0%, see our note on driving after alcohol-free beer for the small amount of extra nuance there.