I really don't know about Jersey records, the marriage just popped up on Ancestry, but if you'd like to tell me who/what you're looking for I'll happily have a look.
It was more a general query rather than a specific, since you had kindly turned up Beatrice Agnes for me. I can access Ancestry in various libraries.
I would be interested in anything more on this Allardice Family. I know that William Alexander died in Jersey on 12 November 1897, but not exactly where, and I have at least two, possibly three, of his children still to find. (I have read that there were 13 children, of whom 4 were sons; so far I have found 5 sons and 6 daughters). I have his birth and that of his wife Helen Guthrie, and I know that Helen died in Kent in 1928.
As for the known children
Helen Maria married William Hamilton Ritchie in India in 1896. They had one daughter, who married but had no children as far as I know. William died in 1901, and in 1902 Helen Maria married Everard Milman Playfair. They are said to have had one daughter but I have not found anything about her.
Mary Isabella married Adam Brack Boyd in India in 1888. I think he must be the one who remarried in 1913, describing himself as a widower, but I have failed to find a death of Mary Isabella (unless Adam was lying!). I don't think there were any children - or if there were I have failed to find them.
William Patrick died in India in 1910, unmarried.
David Onslow died in South Africa in 1960. He married in South Africa in 1905 and had one son.
Alexandrina Colquhoun married John Roland Errington in India in 1893
Laura Winifred married Dugald Alexander MacMillan in India in 1896. I have plenty information about her descendants.
Duncan Alexander, born 1878, probably died in infancy, but I have not found any information about that.
Beatrice Agnes married Bertram Vernon Mitford in Jersey in 1900, as you have kindly told me.
Florence married Ernest Bolden in London in 1919. I think he died in 1952 and she in 1965. I have not found any children.
Harry was killed in France in 1916. I have read that he was married, but have not found any evidence of a marriage.
Colin McDermid was killed in Flanders in 1915. I have no information to suggest that he was married.