NOTE THIS IS A FLUID MATTER and there are different rules/approaches every day so one needs to keep on top of the matter by visiting the proper information sites EVERY DAY.
As an example, today (Dec 16th) FRANCE has toughened the rules for UK visitors:
"from Saturday, people coming from the UK will be required to have a
negative Covid test of less than 24 hours. Once in France, they will
have to self-isolate for a week, but that period of quarantine can be
ended after 48 hours if they have a second negative test once in
France. Compelling reasons will be needed for entry, which includes
visiting family - but not tourism or non-urgent work reasons. The new
measures apply to everyone regardless of whether they have been
vaccinated."
Source: BBC
In the case of Catalonia, this is (should be!) the most up to date source of information: canalsalut.gencat.cat or else if travelling anywhere in Spain www.spth.gob.es