Is it really okay to ask my friend that I want to spend a vacation to his country?

I have a foreigner friend that invited me to spend a vacation to his country, UK. We are good friend while he was here in Philippines. He said he'll be paying for my airfare but I just need to process my UK Tourist Visa and Pport but sh*t happens when Oasis Airlines got bankrupt last April this year and the travel plans were messed up. Now I'm wanting to have my London vacation but how will I open up the subject? Note we only communicate now through email. Is it really okay to ask him to pay for my airfare because thats what he told me even before he was here.