Depends on what the letter is about, the address at the bottom of that letter is his main corporate office, they will just redirect the correspondence to the relevant part of the business.
If its about VS in particular, send it to Sir Branson's Office at the normla Virgin Atlantic address (let me know if you don't have this). He has a team of people there that will respond to the letter, Sir Richard has been known to read a a nubme rof the letters himself and reply
Thanks,
Richard