Harvard doesn’t come with a template with “statute” so you need to create one. I have added the Statute template, and looked at what fields are already there in the reference type for statute. Seems to not be intuitive, as you noted. I attach a zip file with a modified Harvard with a template for Statute, and some jpgs of screen shots of what I tested.
I used
name of Act (as title)
Year (as Year)
Code (as 165)
used [online] as hardwired in style, (but if it isn’t always - one would have to define a field to hold the alternatives and replace this with the field name in the template)
Country (as “place” or London, in this example)
Publisher (as The Stationery Office in this example)
the Access Date as the whole of 15 May 2018 in your example
URL to the Available from: … (I guessed this should be a URL based on the fact there was an access date)
Harvard-withstatute.zip (236 KB)