"As far as I know it is the movement to divorce church from state."
Absolutely correct!
"It is ridiculous to have the monarch as head of the Church of England,"
Now while that may be true as viewed today, it was certainly necessary at the time the Church of England was created as in so doing England was spared the bloody sort of Reformations that occurred in Europe and in Scotland.
"..and for the constitution to forbid the heir to the throne to marry a Catholic when the population consists of so many different religions is lunacy."
Again, it appears nonsensical today, but, taking into account the political machinations of both France and Spain up until the time of the French Revolution, this constitutional 'safeguard' was viewed as being essential if England and latterly the United Kingdom was to remain free and independent.