score:6
According to the UK Foreign office travel advice website https://www.gov.uk/foreign-travel-advice/france
UK nationals can enter France if they have completed the necessary βattestationβ (see above) confirming that their travel is an absolutely necessary journey.
Therefore, it seems that although you are just passing through France, because you are entering French territory then you must still comply with these restrictions.
It may be best to contact Eurostar and talk to them directly to find out more details for your situation and how they could help.
Edit: As of 15:00 on Friday 20th March Belgium has also closed its borders for non-essential travel.
Source: https://www.politico.eu/article/belgium-closes-borders-for-non-essential-travel/