Dear friend,
I would really like to stay over longer. Before the wedding will be more comfortable for me as my work starts the day after. I can arrive three days in advance. Will that be all right? And I want to see as many things as you can show me in London. Something only local people know of, maybe? I definitely won't mind sharing the room with you.
By the way, I wanted to ask you about the gift. What does your sister like? Or if you did not buy anything yourself yet, maybe we can buy a present from two of us? Let me know what you think about it.
Can't wait to meet you already!
Sincerely yours,
....