Source? I've seen calf brain on the menu plenty of times.
(Source: I live here, I'm old enough to remember it dominating the news cycle for months.)
The ban was introduced on December 16, 1997, after government advisers reported small risks that small nerve endings near beef bones and bone marrow might be infective. The ban included cuts such rib roasts and oxtail, as well as soups and stock cubes made in Britain from beef bones.
The ban was lifted Tue 29 Nov 1999, but other bans on using as food more risky parts of cattle - brain, eyes, tonsils, spinal cord, spleen and intestines - remained in force, as did ban on use of bones in manufactured food and cattle more than 30 months old are banned from the food chain.
Things have been eased further since 1999 but I'm not sure we can get brain on the menu in the UK...
The prion protein is expressed in the central nervous system, which isn't legal for human consumption. Also, ingestion of meat exposes it to stomach acid and digestive enzymes before it gets into the consumer's circulation -- enzymes that break down foreign proteins into (harmless) amino acids.
In contrast, accidentally injecting yourself with material from an infected brain is about the most direct possible infection route.