Dear Mum and Dad,
Thanks for your letter. Sorry for not answering earlier. I've been in Great Britain for some days and I really like it. Yesterday my mates/friends and I went sightseeing in London. We saw lots of antic architecture which is famous all over the world. For example we saw Big Ben and Tower of London. It was really amazing! I've already been in London Eye and have seen breathtaking views. And guess what - I've even meet new friends there!
Sorry, I must go now. It's too late here.
Drop me a line!
With love,
/your name/