70p a pound for Spanish onions strikes me as about right tbh. The ordinary brown ones are suspiciously cheap at supermarkets - probably a loss leader.
Sounds fairly darn reasonable to me too tbf - large specialist onions command a considerable premium over their smaller and much easier to grow relatives. Pretty sure you'll pay roughly the same, if not a little bit more (£3 for a net of 3?) for similar numbers in the supermarket - Sainsburies 'Taste the Surplus Branding' Vidalia onions iirc. I'm far from an expert on the Bree/Brie/Biriani situation, but the cheeses generally seem competitive compared to their quality ime. Better selections elsewhere admittedly, but at generally much higher prices.
May I add to the chorus of approval for A&C with a giant hell yeah while I'm here too. Fine store, lovely people and one of my most used stores in Brixton, for more years than I care to mention (or even want to think about). It's just a really good all round store, about as far from an unapproachable and 'saw you coming' deli as you can get.