Consider, as soon as your student visa comes to an end*, your wife's partner of a student visa, which depends on your student visa, will also end.
*I do not know - you would need to talk to INZ about this - exactly how your early exit from the PhD course with an MPhil would affect your already granted student visa, nor whether you would still be eligible for a post-study work visa.
IF you are granted a post-study work visa, your wife should be eligible for a partner of a worker visa.
If there is any gap between your being a student visa holder and a post-study work visa holder, there would be a gap in visas on which a visa for your wife can depend. She will not be covered by her same visa throughout whatever changes you decide to make.