I can offer you a piece of the puzzle:

In 1854 the American (bear with me...) journalist Fitz Hugh Ludlow wrote a book called The Hasheesh Eater that detailed his experience with marijuana. In particular, Ludlow describes his withdrawals from the drug after a long period of daily (ingested) use, describing horrible nightmares and other symptoms, though no physical ones. Incidentally, this is consistent with the idea that marijuana is not particularly physically addictive, merely psychologically habit-forming. The Hasheesh Eater was likely influenced by Thomas deQuincey's Confessions of an Opium Eater, an English work of a similar theme from 1821.

The form of marijuana available to and enjoyed by Ludlow was a medical tincture, known as Tilden's Extract. This was the substance's American name. This tincture was first developed by an Englishman living in Edinburgh named James Edward Smith earlier in the century.

So, perhaps those who are more familiar with English Modernism can say for certain whether Woolf and others in her circle consumed cannabis. But cannabis was generally known and available in the U.K. well before the turn of the twentieth century, at the very least.