In that brief heyday of British horror that lasted until the mid-70s there were a few films in a similar vein to 'Witchfinder General' that explored more domestic themes rather than supernatural forces or monsters. The most famous and enduring of these has to be 'The Wicker Man', the unnerving and brutal classic of that brief folk-horror genre that may well have been British horror's high-water mark. But what were the components of the film that made it so eerily recognisable to the British viewer?
One obvious factor is the film's setting in a 'modern' (or at least contemporary) Britain. Although many British horrors are also set in the here & now their use of magical, monstrous or otherworldly themes mark them out as unrealistic (and there's nothing wrong with that). These elements were entirely absent from 'The Wicker Man', which is why it has such an eerie plausibility.
It also addressed many themes contemporary to it's 1973 release. The upsurge in interest in folklore, alternative religions, new age spirituality, rural living and communes that came in the wake of the 60s counterculture are all present and correct, and are not presented in a particularly positive light. The 'dark side' of the hippy dream in the post-Altamont era, the bad trip on the reverse of drug culture, and the social disorder of 1970s Britain all inform a view of alternative lifestyles that is cynical, pessimistic and nightmarish. We can recognise a type of Charles Mansion/'Helter-Skelter' distortion of the British folk dream of the 60s at play here.
Another theme in the film is the conflict between the centralising forces of modernity and a wild, primitive periphery. It is clear that the law enforcement agencies of 'the mainland' regard the inhabitants of Summerisle as being within the reach of justice, whereas the locals regard the mainland with suspicion and as an arrogant imposition. It paints a picture that in the parts of Britain that mainstream/mainland culture has yet to reach people are still performing barbaric, lawless acts that seem ridiculous and quaint to 'us' (the viewer). The existence of werewolves in the lawless wilderness of the Yorkshire Moors in 'An American Werewolf in London' is another example of this approach. In this way the film is a companion piece to American films such as 'Deliverance' with their unflattering portrayal of hillbilly attitudes towards 'city slickers'.
The other very important theme in the film is religion. What 'The Wicker Man' does with religion is very original: it portrays a conflict between an established religion (Christianity) and a revived, pagan religion that has been imported into the island. This sets up several interesting areas of conflict: such as Howie's presumption that Christianity holds sway on the island when it does not and his arrogant dismissal of the local 'customs'; the battle between a 'real' religion and one that has been essentially invented by Lord Summerisle for reasons of simple expediency; and the idea that if you actually believe your religion, what do you have to fear? And even if (like Lord Summerisle) you don't believe it, aren't you entitled to practice it regardless?
The notion of a free trade in religious ideas is also interesting. Lord Summerisle's supposition that 'your God had his chance and, in modern parlance, blew it' means that a demand for certain types of religious ideas supersedes others; the island didn't need Christianity and instead needed the invented 'old' religion as it was sexier, more relevant, and more interesting. What this boils down to is an idea that all religious ideas are of the same essential value, and in a competitive marketplace some will gain an edge. Howie believes in a universal, accepted truth, Lord Summerisle believes or pretends to believe in something he has essentially invented, and in the film these fundamentally amount to the same thing.
In that way all religious ideas are given a level playing field; it really doesn't matter what you believe in, or whether you really believe in it, as long as you recognise it only has the same value as the beliefs of others (whatever they are). This is a very modern, multicultural message – tolerance, open-mindedness and humility are the key to avoiding being made a sacrifice to the ancient gods in a giant wicker man. Don't take yourself or your religion too seriously. Zealots beget zealots. And do not dismiss the sincerely or insincerely held beliefs of others...or, at least until you get back to the mainland.