Purchasing an iMac but pay every month

Apple offers financing for a lot of their products. Typically you will see a link above the "Select" button for a given product on its page. See the image below from the Mac Mini catalog page...

The financing may not come from Apple directly, but it is an option.

It appears that you are in the UK, this financing option is available from Apple, if it is a business purchase you can also lease Apple hardware.

However, depending on how long you want to spread the payments over you can look for an interest free offer on a credit card, usually up to 12 months so you can make a big purchase then have 12 months to pay it off, as long as you make the minimum payment each month you don't pay anything extra.