where in BKK can one get a cut thoat shave with the hot towel?

If you mean by "cut throat shave" that where they use the strait razor then mostly every barber in Thailand does that.

Haircuts/shaving is a real treat for me in Thailand as they do the massage thing, finger pulling, remove hair from your nose, ears, eyelashes etc. and I feel like a new man every time I go in. I pay 80 baht for the works including the shave.

If you go in just for a shave and no hair-cut maybe less.