There is no such rule for the UK. Immigration Rule V4.2 states “The applicant must satisfy the decision maker that they are a genuine visitor. This means that the applicant:
(a) will leave the UK at the end of their visit; and
(b) will not live in the UK for extended periods through frequent or successive visits, or make the UK their main home
It means maximum 6 months per entry. It means Nothing more. It means Nothing less.
There’s no December January in this.
Everytime you are granted entry and if the stamp says 6 months then you can stay for 6 months. Then you have to leave. You can again come back after that and be granted another 6 months if they decide to admit you again.