OK, the link above tells you what you need to know.
Read the bit on Risk Assessment. That part is about ensuring there is minimal chance of damaging the unborn child because of the type of work done. Whether that can be used as a reason to reduce the hours, I'm not so certain - it is more about reducing certain tasks that could be hazardous. It rather depends on the work that your partner does.
Your partner can challenge it and take it through the grievance process. Obviously contact the Union if she is in one. If the grievance process fails, one option could be to discuss it with ACAS. She doesn't have a right to reduced hours - but the company has to operate in a non-discriminatory way and with H&S in mind. And show that it's decision-making process is reasonable. If she does challenge they may just decide to cave in.
As for the rest of your observations, I'm afraid the company is operating in line with the minimum requirements of the law regarding timing of re-employment and pay when off. She is entitled to apid time off for anti-natal care.
Many employers do more than that, but there is no obligation.