Hi,
Please do this.
in SU01 for a user assign the custom role you had created. delete all other roles and delete SAP_ALL profile from profile tab in SU01
Now go to webui and try to do what you want to do ( create PO or view company codes ). As you said it won't work. now go to SU53 transaction and click on "Display for different user" icon at the top. enter user id through which you have logged into SRM webui. You will find some authorisation checks failed here. Add those authorisation objects to your custom role.
thanks,
sankar