i am US green card holder and I have a 6 month visa for UK issued in India for 6 months from 15th Oct,2018 to 14th April, 2019. 1. I visited UK from 15th Oct to 16th Nov, 2018. Now I am residing in US, I need to visit UK again during April 1st to June 10th 2019. Can I apply for extension of my visa before April 15th in UK?
Leaving aside the question of whether you can extend your visa, the fee for doing so is an exorbitant £993. Even if you could, you wouldn't want to.
Instead, you should apply for a new visa before you leave the US.