I suppose if you’re not absorbing it through your gut, it wouldn’t matter how much you take orally, it’s never going to do any good.
Maybe it’s a malabsorption problem due to your ulcerative colitis..not common but not unknown. When I was diagnosed with Crohn’s disease a few years ago, one of the first things my consultant did was write to my gp and tell them to start me on b12 jabs immediately, which they did. I found an immediate benefit and have them every 12 weeks, by about week 10 I start to feel the lack of it.
As your gp has agreed to give you the jabs it will be interesting to know if you feel the benefit, despite your blood test saying you’re not lacking it.