To be allowed into the UK as a visitor, border officials must be satisfied that you are a “genuine visitor”. One warning sign is “frequent and successive visits”, which may indicate that you are making the UK your “main home”.
What there isn’t is a hard and fast rule that a visitor can’t be in the UK for more than six months in total in any 12-month period. Home Office guidance says:
There is no specified maximum period which an individual can spend in the UK in any period such as ‘6 months in 12 months’. However, if it is clear from an individual’s travel history that they are making the UK their home you should refuse their application.
In this podcast, John Vassiliou of McGill & Co talks about the significance of this aspect of the visitor rules. He tells me that, even though frequent visitors don’t need to obsess about going over a six-month total, it’s still worth keeping tabs on your cumulative length of time in the UK.
The discussion is partly based on John’s recent article on the subject, which is free to read: There is no 180-day a year rule for visitors to the UK.