[Allegedly...] A US visitor can stay in the UK for 6 months in any 12 months,does anyone know when the 12 months start?
The VWP is an American thing and does not have applicability outside of US law. The UK has a Visa National scheme which operates differently. Having said that, we can turn to the Home Office Visit Guidance:
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.
What's pointed out in the guidance is there is no upper bound on the time a person can visit. It's not regulated in fixed time intervals like say, VWP. It says:
"...if it is clear from an individuals travel history that they are making the UK their home you should refuse the application..."
So an Immigration Officer could make that determination at 2 months, 3 months, or whenever, even in a day. A great rule of thumb they like to use is "
Visitors should be doing visitor things." So a person who can account for their time as a genuine visitor will have no problems. Also, they will cap off any given visit to a maximum of 6 months, but people do make back-to-back visits by successfully explaining what they are up to.
The rules (i.e., the law) is at Appendix V.
In general a "XX days within a year" type scenario for visas/visits begins the day of your first entry into that country. So if you start your trip on 1 June, your year runs to 31 May.
But I don't think the UK has a fixed 6 months per year rule, rather they have a "not live in the UK through successive trips" type policy. So if you tried two six month trips close to each other the border official might feel you are trying to live there and deny the second entry.