What you're looking for is consumer finance or often called retail finance.
There are loads of companies which offer this, but most will have a minimum turnover requirement at least. You may be able to find one which will help you out if you look hard enough.
What is the price point for your product?
There is an option to offer finance yourself - I believe you can do this without regulation as long as you don't charge interest, and you spread it over a maximum of 4 payments. Worth checking out the FCA website to confirm that. It's then up to you whether you take the risk of supplying the product before all the instalments are paid or withholding until all payments are made.
Presuming your products are probably in general less than £2,000, the kind of customers who can't get a credit card or save up the money, are probably the kind who you wouldn't want to risk extending credit to anyway, so definitely don't risk having to go and repossess someone's windows when they miss a payment. That would be a very sticky situation indeed.