Dear Mr Brown,
How are you? I am now back to my home in China. I would like to thank you for the good time and your help while I was doing my one month exchange student activity in the UK. You had been a good friend and my English teacher as well. I really did learn a lot from you and your family when I was staying in your home, which made me feeling that I was nearer to my own home with your family's warmth. I enjoyed every single moment of my stay there. How I wish that I could stay longer, and I would like to visit you and your family again in the future and hope that you can come and visit me here in China as well. You will be welcome by my whole family too. In the mean time, please keep in touch.
A friend of yours
Lihua