Unemployed disabled with good credit score. How can I start paying for my purchases in installments?

I'm an unemployed disabled (blind).

I have a good credit history, but no one is going to lend me money.

My total monthly income is about £1100, which consists of all the benefits I receive.

I have a Zilch credit card which has a credit limit of £400.

But if I want to buy something over £400, I can't pay in instalments.

As I understand it, the reason is that I'm unemployed.

I wanted to adapt to the credit system and obtain self-employed status. Yes, I still won't have an income, but at least I won't be unemployed.

But as I was told, I still have to report my income and it may not work for me.

Are there any ideas on how I could change this to be able to pay for my purchases in installments? Or is there only one way, to look for a job?