Piecing together various sources, it is clear that there was a no-passport agreement between the United Kingdom and France from 1961 until 1984 and that, even after the termination of this agreement, British citizens were able to use a British Visitor's Passport (in addition to the standard 'full' passport) until 1995.
Between 1961 and 1995, the UK issued from Post Offices
a cheap, simple visitor’s passport with which Britons could enter 25
This excerpt from Tourism and Political Boundaries (the preceding text can't be seen in google books but it evidently refers to the no passport agreement) gives more details:
...allowed British citizens to cross to France for short excursions
and French citizens to Britain using only unofficial identity cards,
which were issued by travel agencies cross-channel transportation
companies without any kind of background or citizen checks...these
no-passport excursions were abandoned by France in 1984.
One could thus travel even without with the British Visitor's Passport (BVP). One of the documents the above passage is most likely referring to is the British Excursion Document (BED) which was valid for 3 days and which could be used for day trips to France (cost £2.50). This Home Front article in the Daily Telegraph gives different conditions for the BED (perhaps from a different date):
...you could travel to France on a British Excursion Document, which
lasted for one month, cost just £3 and was valid for return trips of
up to 60 hours from the same air or sea port.
Confirmation of the termination of the agreement in May 1984 concerning non-passport documents can be found in the French Archives Nationales, ref: circulaire n° 84-214 du 17 juillet 1984.
However, it was still possible to travel without a full passport as
Later in 1984...a new form of visitors card was issued, which enabled
British residents to visit France for periods under 60 hours.
The BVP also continued to be accepted until 1995:
France and the United Kingdom have mutually agreed to discontinue, as
from 1 January 1995, their bilateral agreement of 14 February 1961 on
the basis of which the British visitors' passport was accepted as a
valid travel document.
Exchange of Notes between the Government of the United Kingdom of Great Britain and Northern Ireland and the Government of the French Republic extending the Agreement of February 14, 1961, to British Visitor's Passports issued in the Channel Islands and the Isle of Man (pdf file)
Traités et accords de la France