Since Brexit, UK passport holders travelling to Schengen Area countries are subject to the 90/180 day rule. This means you can spend a maximum of 90 days in the Schengen Zone within any 180-day rolling period – regardless of how many countries you visit within the zone.
The 27 Schengen countries count as a single area for this calculation. A week in France, a week in Spain, and a week in Italy uses 21 of your 90 days. Exceeding this limit can result in fines, deportation, and future entry restrictions.