Basically all employers can do whatever they like. Your choice is to accept it or, reject the terms, raise a grievance, go through all the grievance procedure, and then, if your employer still says tough, It's happening, your remedy is to resign and go to an employer tribunal.
You'll only win if the breach (and it is a breach, since you rejected the offer of the change from weekly to monthly pay), is considered to be material. What's that? Would you have taken the job 34 years ago if you'd been told - you get paid monthly, take it or leave it! If no (and the Court believes that) you win (sort of). You're out of a job - though you'll get compensation for your loss - which you have to minimise by trying to find another job.
My advice? Accept being paid monthly. I used to be paid weekly and thought it was the dugs baws. But trust me, monthly pay is much better and easier to budget with. I get paid mid-month, the bills come off by DD next day, the rest is all mine.
Some battles in the workplace must be fought, others avoided. This falls in the latter group.