Pasta and bednobs are both correct. GPs receive a large part of their income on a points system - the Quality Outcomes Framework - where they have to report on the number of patients registered on their lists under a range of conditions, what they are doing about it, how many are controlled by medication, etc. In addition, every prescription which is filled by a pharmacist is returned to the prescriptions pricing authority, who reports back to the PCTs who currently monitor all prescribing patterns - anything untoward would be quickly picked up.
However, in the past this was not the case - drug reps used to be able to fund doctors in all settings to go on expensive freebies. The new ABPI regulations firmly stopped all that - reps can't give out pens with a drug name on these days! We still work with the pharma industry but it has to be strictly non-promotional, they are not allowed to stand up and talk about their product, sponsorship has to be much more philanthopic and educational these days.
To answer part of the OP's question - yes, if the drugs are prescribed and patient results are improved, the GP practice will receive improved annual income. It would be a rare case these days, IMO, where an individual doctor gets away with taking backhanders from the industry.