You don't need a IBMS accredited biomed degree, the majority of unis don't even offer it.
It's definitely easier to get a first at a lower ranked uni than one higher up. Although, the higher ranked unis will give more firsts out than the lower ranked ones. However, I would go for the better uni if it's something you will enjoy more/have better job prospects from. You're not guaranteed a medical school offer just because you have a first. While, if you went to a 'better' uni, you're more likely to have access to more extra-curricular stuff you can put in your PS etc, you'll have more support, more things to talk about, be involved in cutting-edge research, the list goes on..