As others have mentioned - the regional J is not the best but not the worst. I personally its annoying for an overnight flight - but perfectly fine for a day flight. The downside is they fly a dense regional 777, so you will be in a 2-3-2 config... which is slightly annoying in the middle for obvious reasons (back in the day it was a 747 service with the old coffin seats)
Its also worth taking the direct service to CTS rather than changing in TYO in my view.
And yes you get more TP on HKG-CTS... im exploring something similar myself to get my TPs up for the year...
On another note, even though im baec gold i have not really flown BA all that much

maybe one day there will be a post on the other forum "baec gold flying ba for the first time" lol..