Most EU citizens (including the French) can travel on a ferry from the UK to France without a passport. That's because most of the EU's 27 member countries issue their citizens with identity cards, which are valid for travel anywhere within the EU. If the UK's coalition Government hadn't abolished Labour's plans to introduce ID cards (on a voluntary basis), there would be a cheaper alternative than having to pay for a passport for UK citizens wishing to travel to any other EU country.
Further, the EU has had a long-standing commitment to introducing document-free travel within the whole of the EU. 25 of the member countries are either already fully signed up to the 'Schengen Travel Agreement' or will shortly become so. So anyone (even non-EU citizens) can already travel without a passport or ID card between, say, France and Spain or between Portugal and Latvia. (Airlines or other carriers might require ID for security purposes but that's an entirely separate matter, with many carriers accepting things like driving licences anyway). Only the UK has refused to sign up to the agreement, with Ireland being forced to stay outside it because of the open border between the Republic and Northern Ireland.
That leaves UK citizens needing a passport to travel to any non-UK destination, except to the Isle of Man, the Channel Islands and Ireland. Ferry companies and airlines have no discretion in the matter.
Between 1961 and 1995 British citizens could obtain a one year passport (for much less than the price of a full passport) which was valid for travel to many European countries but that was withdrawn because it didn't conform to international security protocols. At one time there was also a (very cheap) identity card available at ferry ports, which was valid for short visits (up to 72 hours?) abroad but that was also abolished because of security concerns. As stated above, these days (for UK citizens) a full passport must be held by the traveller.
Chris