My friend is getting married next year and has been going through the same process-in which area do you want the hall,how many must it hold and will there be hot food as this will limit you to halls with kitchens-be prepare to have to scrub them though!!
Southwark council have a list of tenants halls detailing whether they have a kitchen, bar and how many they hold on their web site.
Locally you have the Fashion & Textile Museum and Delfinas - both Bermondsey Street, Glaziers Hall near Southwark Cathedral or the walkways above Tower Bridge. All very unique spaces for hire but I have no idea about costs. I think all the contact details are all featured on this website.