Did they suggest that you pay the IVA over 6 years? This is usually required by Northern Rock, and what they have probably spotted is that over such time period, without the costs of the IP, you could probably pay them in full.
Providing creditors agree to cease charging interest, and you stick to regular payments via Payplan, I feel that you can repay those debts in full, and if Northern Rock get a charging order that could be helpful in that they may not pursue you for any ongoing payments, leaving you with more money to fund the repayments to your other creditors and therefore get them paid earlier.
Regards, Melanie Giles, Insolvency Practitioner