There is widely considered to be a shortage of opiates for medical services around the world, and this is in part due to the very, very strict controls on growth and distribution of the product. This is in part why the cost is as high as you say there.
We seem to be getting a lot of stuff wrong in this area....
I'm guessing a GBP 80 (1/16 oz) per day habit could drop to around GBP 10-15 (not really sure about purity, but assuming adulterated to 10% would mean 0.1g street Heroin is equivalent to one of these pills, so that would be the equivalent of 10g or over 1/4 oz which would be GBP 160 probably)