UK visitor visa allows you to stay for 180 days for say a 2-year period between 01/01/2018 to 31/12/2020. Does this mean you need to leave the UK on or both 31/12/2020?
I know that on a Chinese visa the "to" date is the last day you are allowed to enter China. You can for 90 days afterward. I presume this is not the case for a UK visa?