> Ten years ago they used to be fantastic for obscure finds because it seemed people had not caught on, but these days they tend to be subpar, which is probably a major edge the non charitable enterprises have exploited.

Not sure about UK, but in the USA, people have discovered that there's profit in mining thrift stores for quality products and reselling them. Usually online, but also in antique malls*. There are a quite a few apps that make it easy to look up something by picture and see what it's worth.

* not sure what Brits would call this - it's like brick and mortar ebay. Merchants rent out cubicles that they fill with random stuff, and customers check out at a common till when they are done.