I'm not too knowledgeable of the period (The Flashman Papers has stoked my interest), but AFAIK the home service helmet only entered service in 1878; before that some form of shako was still in use in non-colonial settings.
I’m no expert but that’s pretty much my understanding as well. That said, I have read that the Home Service Helmet was trialled for several years before it was formally adopted in 1878. Against this latter possibility, my regular line infantry will stretch credibility by wearing it in 1875 (when the project is set) to readily differentiate them from the red-coated Militia units but also to enable me to use the minis for my 1895 Gothic London project.

Edit: This is a useful reference
