Remember, you are entitled to somewhere to live as part of your IVA, so you won't end up on the street, so the IP can always make provisions.
For instance, let's say you're paying £500 per month and paying your creditors 30p in the £, and half-way through the IVA your car blows up, and you need it to get to work.
Would the IP immediately declare you bankrupt? No - you ask for a new car, agree an allowance (say £1,500) and stop paying into your IVA for 3 months until you've got enough money for the car. Then, they just add the extra 3 months to your IVA, so it now goes for 5 years 3 months.
The only thing that makes the IP worry is if you say you can't return the 30p in the £. That requires a meeting of creditors, and the IP will go out of their way to avoid that.
The key thing is that the car is essential for work - if you happily tell the IP you can easily walk to work, you won't get one. Housing, however, is non-negotiable. You can't work from a tent.
I hope that helps, and good luck!