[rmc]

> Technically speaking the monarch could overrule any objection (afaik the process is not defined by any specific law or regulation, just built on convention)

The UK doesn't really have a constitution in the sense that the US, France, Germany, etc. have a constitution. Power changes all the time, and has gradually gotten more democratic over the last 200 years.

In theory, all power is from the Queen/King, which they give out to Parliament. In theory

In practice Parliament has the power, and if the Queen/King over steps their bounds it's either sorted out by war (Parliament vs. the Monarch (English Civil War in 1640s)), or Parliament kicks out the Monarch and gets someone else in (Glorious Revolution of 1688 when they got rid of the Catholic King and invited the Prodestant Willian of Orange in).