Very interesting. The soldier is wearing what is known as a “lobster-tailed pot helmet”. Although it has earlier Austro-Germanic use, it's most widely associated with the supporters of Parliament during the English Civil War of 1642–1651… Cromwell’s ‘roundheads’.
I’m not suggesting it’s anything like that old and would agree it's most likely late 19th Century but it’s not a brooch depiction I’ve seen before and I would suspect that the historical ‘loyalty to Parliament’ rather than ‘loyalty to the Crown’ connotations are not coincidental.