Dear Tom,
How are you? I am very delighted to receive your letter and I am thankful for your invitation to London,but I can not.I am going to have an important exam.I really want to go but I can not quit the exam.Therefore,I hope you will sympathize and hope that we will meet in London in the near future.Write to me.
I would like to say goodbye now.I am looking forward to hearing from you soon.
Love,
Nga