I work at a charity and they have matresses with bs7177:1996 labels on them. (Bought like 10years ago or something). I am wondering if the fire safety regulations on furnitures has been changed since? Are the matresses still considered to be safe to use? Or do I need to buy new ones? (It still works very fine) 

does bs7177:1996 label still meet the today’s regulation? If it does, I would like to know where I can find the information or something

There is a newer version of the standard, but they don't usually apply retrospectively unless there was a serious flaw in the earlier standard that gives undue risk.

Check out Fire safety of furniture and furnishings in the home A Guide to the UK Regulations page 28.