Download Festival UK – What should I present as an invitation letter for a UK Visa from India?

I will be traveling to UK in June 19 for Download Festival in Donnington. I need to start the Visa procedure for it as soon as possible. I read in an article that I will need to present an invitation letter from UK for the Visa along with other documents. I don’t think the ticket for the concert will come anytime before March, and I do not want to risk waiting till then.

How can I apply for a Visa for the above from India? What do I need to present as in invitation letter?

Thanks for any advice!