I presume you are in the process of failing an IVA.
The IP can only petition for your bankruptcy if the creditors agree as the court fees will have to come from the funds collected in the IVA so far.
I am guessing that the IVA has not been in effect that long by comparison (is it less than 2 years?).
What this means is that your IP will need to supply you with a certificate of termination and you will need to accrue the court fees and submit a debtor's petition for bankruptcy to the court yourself.
Before you do anything however discuss this with your IP first and get their take on the situation.
for a debtors petition you should contact the county court that holds jurisdiction in bankruptcy over the postcode in which you live. If you contact the local county court they will advise as not all of them deal with bankruptcy. They will also send guidance leaflets which are very useful.
Otherwise you can either print off the forms from the web or even complete them online.
Just google Insolvency Gov and click on forms. Then England & Wales link. The forms you need are numbered 6.27 & 6.28.