This seems true in parts, a friend became intrigued by the quality of debate for Britain's membership of the EEC in 1975 vs. Brexit. Two examples for 1975 are ( https://www.youtube.com/watch?v=I2jUYryRYII and https://www.youtube.com/watch?v=_zBFh6bpcMo) and these certainly seem of higher quality.
One hypothesis (similar to Curtis above) was that there were two energised generations, post WW2 and 1960s, who believed that public debate and ideas could change things, and a variety of new-ish institutions and technologies (BBC, TV) could enable that change. They were partially successful, but the energy and enthusiasm dissipated over time.
Another argument is that Britain is reasonably anti the public intellectual (e.g. unlike the French but much better than the US), apart from during certain times of fairly major change (Royal Society/Glorious Revolution; Scottish Enlightenment; post WW2 settlement etc.) and we are going back to our dormant state.
A third may be that the nature of 'public' has changed, and the debate is going on, but not in media forms which stifle and trivialise it (Less optimistic here, the Greens tried hard to change public debate, some say Momentum, certainly we are quite good at up-market PR, brand design and marketing, all of which suck the energy/deflect from certain debates)
In my experience, in private the quality of debate isn't too bad, though a touch cynical