They do know, they just dont care that much.
I worked at Experian for 10 years.
The credit business is just a risk managing business.
Sometimes they will lose out, but they make far more than they write off.
You can actually borrow up to £500 while you have an active IVA.
A completed IVA will lower your credit rating, but it doesnt stop you getting credit.
All it means is some will refuse you, the rest will take you on, but probably offer lower limits and/or higher interest rates.
IVA's get deleted from your credit report 6 years after they start.
(and from the Insolvency Register, three months after they complete).