Do I use 200gb storage every month or could I be buying less for less£’s

iCloud storage is not a monthly usage. You are billed monthly for 200GB total storage; the storage space does not increase each month. It is similar to renting physical storage space or renting an apartment -- you pay monthly but the size doesn't increase.

Use the following instructions to see how much of the 200GB you are using and what it is being used for: Manage your iCloud storage - Apple Support