You can travel to countries in the Schengen area, which France is part of, for up to 90 days in any 180-day period without a visa. This applies if you travel: as a tourist.
The UK is part of the EU but is not part of the Schengen area, a visa to the UK does not give access tot he Schengen area. That is why you had to get a UK visa not a Schengen one to enter the UK and why a UK visa has no use to you to go to Paris. As already stated above, you need a Schengen short-stay visa from France.
British citizens travelling to France on the Eurostar must have the following documents: Valid passport. France ETIAS for UK citizens (from 2024) Visa (if staying more than 90 days in a 180-day period or if travelling for non-tourist or business purposes)
Schengen started in 1985 with just six countries. The UK opted out of the initiative, having much less interest in scrapping border controls because of its island geography.