score:7
There is no such rule, period.
UK law states that a non-visa national, if landed, is to be issued leave to enter for a period not exceeding six months. Usually they're given this maximum period, unless only in landside transit (in which case a 48-hour leave will be issued).
The country does, however, record entries by air as well as exits electronically, and if it becomes apparent to them that a non-visa national is trying to live in the UK through frequent "visits", or perform activities they aren't allowed to, the passenger may face tougher questioning, be asked for additional evidence of bona fide intentions, or be refused entry and sent back to the port of departure.