Wow that’s actually amazing. How did you gain access?
I worked in Jessops years ago and the basements there used to be fairly interesting (few ancient, huge and inaccessible safes!!!) plus a large door at the back and bottom of the cellars which lead to what I think was part of an old street or courtyard. The cellars also connected strangely and were all arched! They were also dry and well ventilated.