I would say choose the Nottingham flying start programme for a few reasons:
1. Accounting is the language of business - regardless of whether you go down the audit route, investment banking or consulting; having 4 years of solid accounting knowledge > self-studying accounting for an interview or to catch up with colleagues knowledge.
2. 12 exemptions form part of the degree. This allows you to be exam qualified as a CA faster than other grads.
3. PwC placements embedded in the degree. This is invaluable (even if its audit). The PwC placements are not in lieu of summer internships, so there is still scope for combining investment banking internships with your audit placements. Essentially, if you game the system right and finish with a 2.1 with minimal modules below 50% (which is the requirement for exemption), you're guaranteed a job - which is something to be taking seriously in this climate. In fact I would say it's one of the few courses worth the tuition fees for international students (at a semi-target).
4. Warwick Business School is an excellent brand and will likely get you offers from banks and professional services firms but again, this isn't guaranteed. Every year, more and more applicants refine their interview skills, perfect their cover letter and CV, form part of a society, have differentiating "relevant" work experience, speak multiple languages etc. The differentiating aspects between students is now marginal. People also have more time to apply due to online classes and so number of quality applicants will increase substantially. The chance of guaranteed work experience at a global leading professional services firm, a job and scope to explore other industries during the four year programme is not something to turn down in this climate. Nottingham forms part of the best of the rest.
5. Investment Banking is still accessible if you don't go via the internship > grad scheme route. If you complete the PwC grad scheme (which you are pretty much guaranteed to due to already reducing the 450 days of work experience (as part of your placements) and also only having to do 3 exams - you can attempt to transfer to Transaction services or Deals once qualified (this is difficult, but doable). Following that, a move to investment banking is likely due to:
-Being cheaper labour - banks recruit accountants due to the pay differentials which can attract undervalued workers
-Technical skills - (ACA exams, (some even complete CFA), extensive audit knowledge, some CF knowledge as well)
Overall, it is a tough decision, but for the reasons listed above, would certainly take the Nottingham offer. The Nottingham brand will not harm your career prospects at all