There are very good historical reasons why catholics are barred from having any chance of becoming the monarch of GB.

Whether these reasons are still valid is debatable, but much of Britain's, and Ireland's and Scotland's, historical troubles were certainly supported or even fostered by the Catholic Church, attempting to revive their old (pre Henry VIII) influence in the realm. That's why catholics are specifically barred from the succession, and no other reason. England even executed one catholic king.
Less justified is the discrimination against women, since the best monarchs Britain has ever had were/are female. But that's just my opinion.

I'm not a historian, I just read a bit, and form my own opinions.