With traditional retail units lying empty, it is often better for the owner to let a charity keep it occupied rent free, than allow it to lie empty and become derelict. But a lot of them do sell very poor quality old lady stuff. We have a particularly good charity bookshop locally, much to the annoyance of independent secondhand bookshops, who have to pay for stock, rent, and local rates.
I did read about one branch of Oxfam in Kensington where they send all the designer stuff, and it seems to get good reviews:
Oxfam, Kensington - Kensington, London - Charities