i think the 6 months max stay is only for guidance, but some immigration officers would be suspicious if you stayed longer, and question why you're really are in the uk and could refuse you from entering the uk
4. FREQUENCY AND DURATION OF VISITS
IDI/ Feb 2006
There is no restriction on the number of visits a person may make to the
United Kingdom nor any requirement that a specified time must elapse
between successive visits. The fact that a person has made a series of visits with only brief intervals between them would not, in the absence of any other relevant factors, constitute sufficient ground for refusal.
It is reasonable, however, for the immigration officer to consider the stated purpose of the visit in the light of the length of time that has elapsed since previous visits. A visitor should not, for example, normally spend more than 6 out of any 12 months in this country (but see Section 3 of this chapter in the case of visitors for private medical treatment).