i don't have experience with going back to the origin, but from what i know it's allowed.
i almost did it once myself - HKG-HND-HKG-LHR-GIG ... (which was allowed but i didn't ticket it)
but ended up doing TPE-NGO//HND-HKG-LHR-GIG//GRU-MAD-LHR/LCY-JFK-HKG-TPE
going back to LHR after Brazil was allowed
call a few different agents - you may get a different answer. some are obviously more knowledgeable than others.
sorry i can't be of more help.