This is from The Connexion
[Article updated June 23, 2021, to clarify rules on sworn statements]
Rules for travel into France from another country
UK to France
What France says:
Fully vaccinated people: You are free to travel to France. You do not have to have an essential reason and do not have to quarantine on arrival.
You must present proof of a negative PCR test taken within 72 hours of departure or a negative rapid antigen test taken within 48 hours of departure. This does not apply for children below the age of 11.
You must also present proof of your vaccination status and a completed ‘sworn statement’ (déclaration sur l’honneur, on the French government website here) form self-certifying that you are not suffering from symptoms associated with coronavirus and have not been in contact with confirmed cases in the preceding fortnight.
Non-fully vaccinated people: You can only enter France if you have an essential reason. Most of the essential reasons involve you having the right to live or work in France.
You must present proof of a negative PCR test taken within 72 hours of departure or a negative rapid antigen test taken within 48 hours of departure. This does not apply for children below the age of 11.
You must also present a completed ‘sworn statement’. This is as above, but also includes a promise to self-isolate for one week.
You are asked to voluntarily self-isolate for seven days upon arrival at a place of your choice (and do a voluntary antigen test) and then later take a PCR test and have a negative result to leave isolation.
Children: The rules for minors are the same as those that apply to adults who have been vaccinated, whether the minor has been vaccinated or not.
What the UK says: Travel from the UK to France is not recommended by the UK government but is not illegal.