Thanks for your answer.
Your latter comment is exactly what happened.
My Partner had an income of around £25,000 when we moved in together last year. My income was around £32,000.
We moved in together knowing I was about to enter into an IVA, and budgeted our outgoings so that any one of our incomes would support our bills. A few weeks later, my partner suffered a mental health breakdown (not my fault

) and dropped down to essentially less than part time.
So, its household income. I absolutely dread the day she goes back to work full time as the financial loss to me will be massive.
Your point about "household" is exactly my thought process. Its still cost "us". In addition and IVA is based on year long monthly payments, the idea that they could ignore a bill because its paid at once rather than spread out strikes me as bizarre. We pay our council tax in one hit too, but they happily accept the yearly payment.
The question is, how would they know? I can evidence the bills and cost, I cant imagine for one moment they are constantly checking my credit file every month to see if its ended early. Ultimately, the cost to me is exactly the same....
Also confused why they'll take my partners income but refuse to acknowledge that she has a vehicle, running costs etc.