The NHS tries to discourage the prescribing of "branded" drugs where possible, simply because they're more expensive and we (hospitals etc) are all supposed to be saving huge amounts of money. Often un-branded drugs are exactly the same as the branded versions (nurofen and ibuprofen for example, nurofen costs upwards of ?1 a packet and the same number of ibuprofen tabs in the same strength costs less than 50p) and do exactly the same job.
There are exceptions, some drugs are prescribed as brands because there is a difference in the pills/caps made and it's important to stick to a brand, Lithium Carbonate is one, theophylline is another.
Atorvastatin is only availible at the moment as the Lipitor brand from Pfizer, because Pfizer still have the sole license to make and sell it. However i think that the protected license period is coming to an end, and all the generic drug companies want to get in on the action. Atorvastatin is pretty much the favoured alternative to Simvastatin, but at the moment it's much more expensive. I haven't ever seen a branded Simvastatin pill, in the 4 years i've been working in hospital pharmacy (techinically i've been in hospital pharmacy for more like 7 years, but for the first year id didn't see many drugs, and the 2nd and 3rd were in a children's hospital where there was much call for cholesterol lowering drugs....saw lots and lots of calpol though...)
Rachel