AFAIK, the fast track services require you to present yourself at a passport office in the UK. You can then get the new passport posted to you (usually takes 3 working days) or collect it the same afternoon.
This is what I also understood from the official government website, but I have so far found two agencies who offer to handle the premium service (same day issue) or fast track service even if you are out of the UK at the time....they tell me that as long as you give a UK address and a UK number, there will be no problem. I just would feel more comfortable if someone here had already successfully tried this solution!