Was there an increased interest in 'the spirit world' in the aftermath of the First World War?

There does seem to be some evidence to support this assertion, but the resurgence in the popularity of spiritualism ("a belief that spirits of the dead communicate with the living usually through a medium") actually happened during the First World War and continued well into the 1930s. David Nash, professor at Oxford Brookes University, writes that:

Britons had been drawn to ‘irrational’ ideas and practices in growing numbers long before the Armistice of 11 November 1918. Interest in spiritualism surged dramatically in 1917, as the nation endured a third full year of conflict. In fact, at one point, this religious movement – which promised believers the prospect of being able to communicate with the dead – briefly threatened to become more popular than the Anglican church.

Source: David Nash, 'The rise of spiritualism after WW1' (2020)

Spiritualism, which had come to Britain from the US in the mid 19th century, had seen its popularity decline by the early 20th century, but the First World War reversed that trend. Kyle Falcon, who completed a PhD thesis 'Ghosts of the First World War: Spiritualism and Psychical Research in Britain', notes that:

Jay Winter and other scholars have argued that Spiritualism was appealing to Britons during the Great War because it offered bereaving family members the pragmatic comfort and communication that traditional religions and prayer could not provide.

The aforementioned Jay Winter notes that the interest in spiritualism was evident in other European countries, including France and Germany. He elaborates on the reasons:

During and after the Great War, interest in the paranormal and the after-life naturally deepened. It was inevitably and inextricably tied up with the need to communicate with the fallen. One French observer reversed the point. 'After a murderous war', he wrote, 'who would doubt that the dead would try to communicate again?'18 This became a key feature of the wartime and postwar appeal of spiritualism. It provided a means through which the dead led the way. They helped both to lift the burden of grief borne by their families and to spread the 'truth' of spirit communication.

Source: Jay Winter, 'Spiritualism and the ‘Lost Generation’. Chapter 3 in 'Spiritualism and the ‘Lost Generation’ (2014).

One piece of evidence in Britain is the increased membership of the Spiritualists' National Union, founded in 1901.

In 1914, there were 145 societies affiliated to the Spiritualists National Union (SNU); by 1919 there were 309. In 1932, the newly established Spiritualist and London based journal, Psychic News, announced that there were 500 societies affiliated to the SNU.

Source: J. Hazelgrove, 'Spiritualism after the Great War', in 'Twentieth Century British History, Volume 10, Issue 4, 1999'

In addition, in the 1930s, there were large meetings held:

Both London and provincially based meetings were very popular. The medium, Helen Hughes, regularly addressed an audience of 3,000 or more, while 9,000 people gathered at the Royal Albert Hall to witness Estelle Roberts speak to the dead.

Source: Hazelgrove

Further,

publications dealing with spirits and the spirit world were undoubtedly popular. Sir Oliver Lodge's Raymond (1916) and Arthur Conan Doyle's New Revelation (1918) and The Vital Message (1919) attained the status of Spiritualist classics in the interwar period. Perhaps the single most successful publication was On the Edge of the Etheric (1931), by Arthur Findlay....On the Edge of the Etheric was reprinted twenty-five times between November 1931 and June 1932 and, according to Psychic News, sold at the rate of 500 copies per week. In its first year of publication the book ran to thirty impressions and was subsequently translated into nineteen different languages.

Lodge's book, which related "the joy the author felt on contacting his own dead son" (Raymond was killed in 1915) became a best-seller, the author's knighthood and scientific reputation helping sales along even though the book was criticised for lacking credibility. Conan Doyle, another major celebrity in his time, frequently engaged in public debates and gave speeches on spiritualism.

The popularity of spiritualism was of some concern to established religion, although it wasn't simply a question of one or the other; beliefs often indicated a complicated mix of church teachings and spiritualist beliefs and stories. The Anglican church, with declining attendances, set up a committee to "investigate the subject of communications with discarnate spirits and the claims of Spiritualism in relation to the Christian Faith". Even during the war,

one chaplain told David Cairns, the Aberdeen theologian, 'The British soldier has certainly got religion; I am not so sure, however, that he has got Christianity.'

Source: Winter

The Catholic church was at least equally hostile:

...the distaste with which Catholicism traditionally viewed Spiritualism was now accompanied by marked signs of anxiety, and special campaigns were initiated to combat the perceived threat Spiritualism posed to religious life. In 1926, a society called the Catholic Crusade Against Spiritualism was formed. Its founders feared that Spiritualism accounted 'for considerable leakage among Catholics', and proposed by means of lectures to 'expose the dangers which follow upon the practice of the cult'.

Source: Hazelgrove

Despite the exposing of many frauds, the most notorious perhaps being Helen Duncan, for many spiritualism connected in a way which provided them with comfort. As David Nash puts it, at the end of the war,

Some fervently sought a return to normality – to the comforting certainties of the prewar world. But others exhibited a starkly different outlook. They reasoned that it was those comforting certainties, the very institutions that had dominated Britain in the countdown to 1914, that had led the country into a catastrophic conflict. Their reaction to the horrors of the war was to reject rationality in favour of something altogether stranger, something that encompassed witchcraft, spiritual renewal and communication with the dead.