Hi Gemma,
Here is my two cents.
Firstly, The only uni I know of that will place you really well out of the ones you listed is Glasgow, so I advise you go there. Sheffield second.
Once you get there, join as many clubs and societies as feasible and try to get board positions ASAP. Def. join GUTIC and try to get a position there if you can... Same with the poker society in my opinion, as well as the business club.
Differentiate yourself somehow by doing something other wannabe IBers don't... Join the citizens advice... Become a charity mugger. Whatever.
Apply to spring weeks for everywhere.
Apply to the big four as well as all the Scottish asset managers for internship positions... They seem to favour Scottish grads from what I've seen. Also apply to the larger oil and gas companies... Anything with a recognisable brand.
After getting spring weeks, internships as above, start applying to internships at banks. As has already been stated by someone else, Scotland is good as it gives you one more year, one more chance.
In your second or third year at Glasgow, consider changing your degree to accounting and finance or economics.
Apply speculatively to lots of the smaller firms that do IB type work. Quayle Munro etc.
Stop reading the student room as it will discourage you since most people here aren't sure what they're talking about. Start reading wall street oasis. Read other sources of IB related stuff and make it your passion or you don't stand a chance.
Get a first in your degree.
In my experience, taking the round about route as above it your best shot, but if you can, save yourself a lot of hassle and just get into a target. Remember that you need to be as good as everyone else applying, but you also need to differentiate yourself somehow.
(caveat: former Glasgow uni student. Not working in IB, but corporate development.)