Upvote:5
In the UK, as in the Schengen area, you may only remain while your visa is valid. Six months is the maximum duration of validity for a standard visitor visa, but if your visa has a shorter period of validity you must respect that.
If your visa has been issued for a shorter period, you can apply to extend it. The total duration of your visit may not exceed six months. Like many UK immigration fees, this is ludicrously expensive, costing £993 for the extension and £19.20 for biometrics processing, for a total of £1012.20.