IVA and bankruptcy both affect your credit rating similarly, although they will tell you it won't. The main reason that they (the company handling the IVA) tell you that it won't is because they get a slice of your money.
For an IVA to work, you need to be able to commit solidly to making regular payments towards your debt.
Basically, your creditors are approached and told "Cesario hasn't got much money, and there is no way you are going to get all that she owes you. The likelyhood is that she will have to file bankruptcy, and therefore you will get nothing. However, Cesario could pay her debts if they were lower. Cesario could offer you £0.20 for every £1 she owes you."
The companies that you owe money to, will say "No way, we want £0.80 for every £1 she owes us"
The IVA company then says "Don't be rediculous, guys. The most we can offer is £0.40 for every £1".
The companies then have to vote on whether they accept this offer. If companies who represent 75% of the monetary value of your debt vote "Yes", the IVA goes ahead, and the remaining companies have to swallow it. If not, the IVA fails.
You need at least £200 disposable income to be able to pay towards the debt, and you won't be able to have any credit from that point on until the IVA is complete (5 years). After that, any remaining debt is wiped off.