Well both are defined by a certain extent, it has to adhere to the HO club criteria, and their guidance makes their interpretation clear.

So no, no one can just go out and start one because Sect 21 of the acts prevents some, HO rules on clubs would pick up others, and .223 wouldn't be deemed within interpretation.

https://assets.publishing.service.go...l_2016_v20.pdf

18. 5

Whilst there is no legal definition of a miniature rifle, other than one which does not exceed
.23 inch in calibre, it is generally accepted that this refers only to rifles firing .22 rimfire
cartridges (see also chapter 6). Persons using the range are exempt from holding a firearm
certificate only whilst using such miniature rifles and ammunition at such a range or gallery.
Home Office approval can only be granted to a miniature rifle club if it can adhere to the
Home Office’s club criteria.