Dear Mr./Mrs. Brown,
Thank you again so much for having me for the past three years. I just wanted to let you know what a great memory I had with you while I'm in London. You have provided such a fantastic hospitality, and it was so nice of you to guide me through my hardest phase in those years.
As you know, I'm heading back to China soon, and I'm not sure when I'll visit again, but I hope it won't be too long, because it would always be nice seeing you and we had such a fun time while we were together. Please let me know of any plans you might have to visit my country. I would love you to visit me and my family sometime and then we could have the pleasure of taking you around.
Once again, thank you for everything. I will always miss you.
Yours Sincerely,
Ming Lee