If the "leave to enter for 6 months" was granted on July 1, 2018 and you only stayed 3 weeks and then after the 3-month entry block you decided to come back for another month that following December, would your exit date remain the same or would the 6 months get reset as it was a separate trip?
1It will be reset.– Augustine of HippoAug 21, 2018 at 6:45
5@Rachel There is no ‘3 month entry block’ within the UK immigration rules. The criteria for multiple entries, whether visa-free or with a visa, is that the visitor will not live in the UK for extended periods through frequent or successive visits, or make the UK their main home. See v4.2 gov.uk/guidance/immigration-rules/…– TravellerAug 21, 2018 at 7:17
Every time they let you in, you will get a fresh 6-month period. Your previous visits are not counted in order to decide how long you can stay.
With the obvious caveat that if this was not a visa free entry and was granted on a visa, then it's either 6 months or the expiry date of the visa, whichever comes first.