I’d advise you to check out the citizens advice centre either go & talk to them or check it out on line as I’m sure they will be able to help you out & it would be good for you to go into the meeting prepared with printed information on the subject.Good luck with your meeting.
They cannot force you not to take it if you have an agreement in place but they may well ask you not to continue with it and if you are not keen on that idea, they may try to pressurize you into it by saying you are really busy. Did you agree in writing (email counts) – if so, get that out and have it at the ready.
I would advise trying to keep things as amicable as possible though, you do not want to get into a difficult or stressful situation with them.
Check the maternity act site for more details or talk to some one in HR quietly about it, in confidence, before your meeting if you can. They can advise you.