This question already has an answer here:
I got my standard visitor visa for UK stamped and it starts from 15th April. But my flight will be reaching London on 14th April 7AM local time(due to my mistake of overlooking dates). I think airline will not allow me to board. I am currently in USA and am a permanent resident of USA with an Indian nationality. Can I apply for change in dates to start a day early or apply for a new visa? What are my options?