I have been to Billingsgate a couple of times as a corporate hospitality event - overnight in London, a 6.30am tour of the market (as it is closing!) and then breakfast in the fish school, followed by fish preparation instruction and a three course lunch with wine before departing with a coolbox full of goodies mid afternoon.
It’s a great day out (I think around £180 per person), but London accommodation and meals would make it quite expensive in total.
I would certainly be up for a trip to Brixham.
ian