How does GP system works?

I would like to know, when we go and get prescription from GP, who is responsible for the cost, I mean the initial cost of the drug come from GP quota or NHS and it does not effect GP?How does GP system works?
things like this oscillate from country to country, in US, it is your condition insurance that covers part or full of the prescription depending on the plans you chose, other countries hold different systems, good luck
Each GP practice have a notional drug budget. Currently it is not mandatory to keep inwardly it. However we receive lots of visits and a large amount of stick and hassle if we continue to exceed it. All prescribing is broken down both into areas and individual drugs. If we are found to be using big numbers of non-formulary or branded drugs we are censured. We can be investigated if the excess is significant.