There is no law which dictates that a person or group of people cannot claim soveriegnity. There is, indeed, legal gounding for the exsistence of micronations, in the form of the Montevideo Convention on the Rights and Duties of States 1933, which lays down four basic requirements for a state to be considered soveriegn:
1. A permanent population.
2. A defined territory.
3. A government.
4. The capacity to enter into relations with other states.
Therefore the H.U.C is legal.