How to pay for my enhanced storage?

You can purchase iCloud + Storage > Upgrade to iCloud+ – Apple Support (UK). This is a monthly subscription and you will continue to receive the storage as long as you continue to say subscribed.

This allows you to purchase extra iCloud Storage, Apple's Cloud Storage service. Please remember that iCloud storage is different to your device storage. You are unable to increase the storage of your device, however, you could use iCloud Storage to enable iCloud Photo Library, saving the high-quality original versions of your photos on iCloud, while keeping them on your device in lower-quality form, saving storage.