Two observations on this:

1. Whatever its history, the X symbol has become so trivialised in the present day that many younger people put it at the end of a text message, facebook post, etc. although they can sign their names, have no religious belief, and hardly know the person the other end. X is being reduced to superficial slobber.

2. If you do enough family history research into marriages to give you a statistically viable sample, you may well find that the use of X provides evidence of gradual increase in literacy. I have discovered that in my small family sample of 52 (from Glos but outside the Forest in this instance) all 20 parties to marriages between 1850 and 1899 signed their names. In the batch between 1825 and 1849, 12 signed and 6 marked. Between 1800 and 1824, 4 signed and 10 marked. A wider survey of this trend might be interesting in terms of both literacy and demography.