The better London firms do tend to recruit from the better London universities, but if we're honest, that's not really what's on the cards here and going to a London uni outside of KCL, UCL, LSE & Oxbridge is likely to do little for your chances over going to Bristol UWE etc.
That said then, go to the university that excites you the most. If you like the look of the course and campus then you can overcome any disadvantage if you just apply yourself. Won't be easy, but it's definitely doable.