Dear Sir or Madam,
My name is (the name) and I bought a ticket for the next concert in London. Unfortunately, it has cancelled because of the illness of the singer.
I had already booked accommodation and flights. I cannot cancel it, but the organisers only refund the ticket price. The budget was really expensive.
If you could give me two extra tickets for the next concert as compensation, I would be very thankful. If it is not possible, I would like to ask for a full refund.
I look forward to hearing from you.
Yours faithfully
(the name)