In terms of getting a job, it won't really matter where you go to uni. Nursing and midwifery education hasn't been based solely in universities for all that long compared to the history of the profession, so the distribution of which unis offer midwifery degrees (whether Russell Group or ex-poly) is largely historical accident rather than a reflection of the quality of the programme. Employers won't care whether you studied at a Russell Group or not.
You should be aiming to go to the best course for you in terms of facilities, placements etc, but honestly, midwifery is so crazily competitive that if you get an offer anywhere (which you have, well done!) I think you should take it. Many, many people get all unsuccessfuls for midwifery and reapply 2 or 3 times before getting in- if you declined your offer now in hopes of getting into a Russell Group next year, there is no guarantee that you'd get an offer or even improve your chances all that much due to the sheer numbers applying vs the small number of places available.
I do take your point about possibly not feeling academically challenged enough at BCU- to that, I would say that you get out of uni what you put in. Many people just do the bare minimum with academic work and don't use their tutors much as a resource outside of lectures- if you really engage with the theory of the course and go above and beyond what's required you can enrich your experience that way.