Many countries have this requirement, however it is seldom enforced unless you are coming from an area with a known Yellow Fever outbreak. It is worth getting an injection just to be on the safe side. Any travelers clinic in UK, or a GP, can do it. Only a travel clinic can get you the card which proves your injection.
I am an Italian passportholder, living in Norway. I have been told that i will need a visa to enter Gambia. I am going on a 8 -days trip, arranged by a travel-agency, so I will be travelling fro Norway. Will i need a visa?