Hi ,
You can apply for visit visa or if your research institute in london send you letter than you can apply for study visa that would best suit you. Good Luck.

Hi there I donot know who you are,few of things which i want to share with you are my two experience in this countrty. You are indeed very lucky to have phd opportunity. First of all make your mind what u want to do in your future would this phd be helpful to or want keep a degree in cuboard. It is hard to achieve phd degree without working thing is that there is nothing difficult in this world you can achieve every thing but you have focus on your goals in life. Nothing gone change by crying, it will result only hard to achieve your goal. In my point of view If you work seriously there is nothing difficult. It is life time opportunity please donot let this opportunity go from your hand ask for Help From GOD. God is always there to help his people. You could not imagine the happiness on your parents face when you will get phd degree at such young age they would be proud of you.
Take care donot leave hope.