If the creditors don't accept an IVA then you can either do a debt management plan or declare bankruptcy.
Have you started the process yet? If not then I suggest you speak to a professional. Visit www.iva.com for a list of companies and reviews and give one or two a ring for some free and impartial advice.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
It is quite unlikely these days that creditors would turn down a good offer - so long as you demonstrated that you were putting your very best offer on the table. Some of them still have specific criteria, but the IP you choose to represent you will be able to give you guidance on the likelihood of acceptance, without any guarantees of course.
you mention that some creditors have specific criteria, i'm guessing maybe minimum dividend type criterion (?). In your firm would you expect any issues with specific criterion to be picked up early on (consultation/admin) or would they only really be identified when the proposals team analyse the case?