I have located an 1851 census for William McAra mistranscribed as McAree, soldier age 21, 33 Regiment, born Stirling Scotland. As an Artificer he could be a soldier and would explain why he moved to Plumstead, location of the Royal Arsenal.
I have found a marriage for a John McAra and Mary Maxwell 27/04/1831 Parish Stirling County Stirling
No Date Surname Forename Spouse Name/Frame No Sex Parish City/County GROS Data
19 27/04/1831 MCARA JOHN MARY MAXWELL/FR1618 M Stirling /STIRLING 490/ 0070 0073
Also found a birth for a John McAra but it looks like the family moved Crieff and had several other children
No Date Surname Forename Parent Names/Frame No. Sex Parish City/County GROS Data
23 16/04/1810 MCARA JOHN ALEXANDER MCARA/JEAN MCEWAN FR1500 U St Ninians /STIRLING 488/ 0040 0201
Any help or ideas would be appreciated, I have tried to find military records but no luck.