You do need that completion letter and don't forget your credit rating will be shot for a full six years from your start and you won't be off the Insolvency Register until they receive their copy and it could take up to three months to come off. Chase things up with your IP.
You may struggle to get a car loan.
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
Until you get that certificate you are still, technically, in an IVA and cannot take further credit. In any event, and this will apply after you get the certificate, too, until your credit records clear up, you will find lenders either refuse or impose stupid interest rates.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk