I assume that there was nothing put in writing by the employer that suggested this arrangement might be reviewed and changed?
Assuming not, your wife could argue that she has an 'implied contract term' that she works the rota as is, because she has been doing so for 8 months now.
It might work and get them to withdraw.
At the very minimum, the employer needs to consult over this (and that means a discussion where the employer explains its side and asks for a response on what issues the employee might have in doing it - not a 'your shift changes next Friday, luv, you're OK with that I assume?'). It also needs to give notice of any change - one week per complete year of employment as a minimum.
As far as I know (and the law was changed a few years ago) the employer has a duty to consult, but it does not HAVE to agree to a change proposed by the employee as the result of childcare requests. But this is different - the employer had already agreed, but now wants to change it back. Try the 'implied term' tack - then mention the potential for constructive dismissal in passing if that doesn't work.
At the end of the day, the employer will get its wicked way over this if it wants to - even if it means being taken to an ET, losing and having to pay some readies over. The issue is how entrenched it wants to get and whether it dislikes the prospect of potentially adverse publicity.