Simple present is used for future when the cause of the future act is some sort of external determiner.  In 'I go to London tomorrow', the intimation is that it has been scheduled, so the speaker must inevitable go.  In the letter-writing situation, however, the speaker has obviously made the decision himself, so the 'going-to' future is the most appropriate future form.

I think this type of construction is mostly used for pre-arranged or scheduled events. Going somewhere is likely to be a scheduled event, so with "go" it sounds natural. Writing to someone isn't usually a scheduled event, but if it was then you could use the construction with "write". For example, say you always write to your mother on the first of every month, but she gets confused and rings you the day before, saying "I didn't get your letter". Then you could say "I write to you tomorrow". Not enormously likely, I admit, but just about possible.