It is important, at this point, to say that an IVA , on the evidence provided, may indeed be viable, but a while back it may well not have been, for various different reasons. On that basis, GP may well have done everything that they can to help you.
However, your debt is always yours, and therefore how you deal with it is your decision and no one elses. Stick with GP (Insolvency arm is Freeman Jones) or twist and go elsewhere. Your decision alone.
I am aware that some of the larger DMP companies will charge upfront fees for IVA's, even if you have been with them for a while, and even a "transfer fee". I don't know, in fairness, whether GP do that or not, but if they do then I would say you should run a mile and find someone who won't charge to look at it. If not accepted then fine, stay on your DMP or consider other alternatives, if accepted then you have the legal guarantees you need, but, either way, at least you will risk (or pay) nothing to find out.
Hope this helps.
Regards.