The specific limits of the Monarch's royal prerogative have never been formally codified, and thus are somewhat ill defined. The Department of Constitutional Affairs produced the below listing of them in 2003, though it isn't necessarily definitive.
- The appointment and dismissal of ministers
- The summoning, prorogation and dissolution of Parliament
- Royal assent to bills
- The appointment and regulation of the civil service
- The commissioning of officers in the armed forces
- Directing the disposition of the armed forces in the UK
- Appointment of Queen's Counsel
- Issue and withdrawal of passports
- Prerogative of mercy
- Granting honours
- Creation of corporations by Charter
- The making of treaties
- Declaration of war
- Deployment of armed forces overseas
- Recognition of foreign states
- Accreditation and reception of diplomats
Theoretically, these powers are held by the Monarch. In practice, these are the powers held by the Cabinet and they're "used" by the Monarch on their advice.
Any of these powers can be removed or otherwise modified by an act of Parliament, which would happen in short order if any Monarch decided to attempt to ignore the Cabinet and seriously utilize their theoretical powers.