The difference between you getting the Prostap and the practice is purely down to cost. If you live in a modern Care Commissioning Group (CCG) - the people responsible for deciding which services their GPs should buy in - then the GP will administer, store and monitor the Prostap regime.
If you live in a backward one, then you obtain your own injection. You can contact your local CCG and find out which services they have bought as they are all different. Supposed to be a NATIONAL Health Service. You can monitor their performance on NHS Choices too.
After a year of complaining, badgering, annoying and attending meetings with my local CCG, they have now agreed to provide the same services for Prostate Cancer as the neighbouring areas so we are all treated the same. The GPs will now provide the drug, administer it and also do the full monitoring from the PSA test.