The King can appoint anyone he likes as prime minister and anyone he likes as lords. The Royal Family only follow a predictable tradition. The King carries command over the armed forces, police, etc. I do not trust an elected official with that role. The fact is, the Windsor family (as they are now called but have gone through different name changes) have been custodians of this land for almost 1000 years and in the last 300 have done an excellent job at keeping it. The biggest threat was Edward VIII who was swiftly removed from power anyway.