虽然不想相信，但翻入境条例网站，条文的确是这样说的。

Requirements for an extension of stay as a general visitor

44. Six months is the maximum permitted leave which may be granted to a general visitor. The requirements for an extension of stay as a general visitor are that the applicant:

(i) meets the requirements of paragraph 41 (ii)-(vii) and (ix)-(xii); and

(ii) has not already spent, or would not as a result of an extension of stay spend, more than 6 months in total in the United Kingdom or not more than 12 months in the case of a person accompanying an academic visitor as a general visitor. Any periods spent as a child visitor are to be counted as a period spent as a general visitor; and

(iii) has, or was last granted, entry clearance, leave to enter or leave to remain as a general visitor or as a child visitor.