They don't exercise much power outside of their ceremonial role but they still have a huge amount of power. Everything the UK government does requires royal assent. While only used ceremonially now, the government requires the Monarch's permission to form in the first place and the PM is appointed by the Monarch. The Monarch has the power to unilaterally dismiss the PM and disband parliament. The last time a Monarch denied assent for a bill was in 1708 when Queen Anne vetoed the Scottish Militia Bill. The last time a Monarch broke convention with the use of their powers was in 1834 when William IV dismissed the Whig PM and cabinet and appointed a conservative PM and cabinet to replace them. The last time breaking convention was publicly discussed was in 2013 to stop Brexit.