You can get B12 tablets and injections on the NHS if you need them (giving these depends on your level though, and the doctor might say to try to increase the amount of B12 in your diet before trying these.)
Sources of B12 in food (if I remember correctly) are things like meat, chicken, fish, cheese, milk etc. Because of this in the UK most people with (diet-caused) B12 deficiency are vegan or eat few of these foods.