On british bank notes it says I promise to pay the bearer on demand X pounds on demand?

X pounds of what? Silver? Gold? How? Notes and coins for years now have not been backed by gold or silver. Does this mean that modern day money in britain is a big con and the money is valuable only because we are told it is? Seems to me like worthless bits of paper and whoever is printing it is cheating us all into thinking it is valuable. Does this make everyone with a national insurance number a modern day slave and employee of the united kingdom corporation?