Well my memory mostly and that of those slightly older than me (around your age it would seem). Despite being taught both at school the weather was always given in Fahrenheit for headline purposes. In the 80s I recall seeing both given as a transition. Do you have anything to back your idea up other then to call anyone who didn't immediately adopt it a fuddy-duddy which is not really a scholarly citation is it now? My memory is that it wasn't fully embraced at the time Adams wrote HH. Without resorting to name calling can you furnish me with media snippets that solely used Celsius?
That linked document is hardly a record of British culture is it? I need to see something that suggests your view is more than the assertion you credit me with.
Furthermore, if this was the meaning of 42 then it could have been arrived at any time prior to when the last draft of Hitchhikers was given to the publishers and more likely to have to been considered in F than C the further back we go.
Update: looking at some videos of TV weather reports from the 1970s they were indeed given in C (with the temp also given in F). I only really started using it in earnest after marrying a European and I have to say I can guesstimate the temperature in Celsius more accurately than I ever could (if I did) in Fahrenheit. Pity we don't have any vox pops from the time accurately recording what the British public actually used.
Hmm. It was written in the 1970s, a decade when Britons still mostly used Fahrenheit so I'm a little sceptical.