I suspect however bad your credit rating is prior to IVA, that's nothing to how bad it gets when the IVA is agreed!!!
Having recently gone through the account switching process myself, my IP advised getting it sorted prior to IVA for this very reason.
Thus, however IVA unfriendly a bank may be, providing you don't subsequently volunteer your IVA status, or go overdrawn/apply for credit, you should be OK. (That said, I have read anecdotal threads advising that some banks routinely trawl the insolvency register and subsequently close the accounts of us insolvent un-desirables!!!).
Case in point, I'm with HSBC (business acc), and First Direct (current acc). Aparently they are the worst of the lot. Will let you all know how long it takes if ever, for them to kick me out once my name appears on the insolvency register.
My opinions are just that: Based on my experience and being a self-employed IVA customer.