Skeeterbd:
Regarding the use of your gun as-is: Best not to until the dent is raised. Although thousands of guns have been shot with worse dents than this. That said, please consider any dent to be a barrel obstruction. The larger the dent, the more serious the obstruction.
Common sense says we should never fire a gun with a barrel obstruction.
Your Greener is a fine one. I'd find a top level smith to fix it -- or even send it back to the UK. Smallbore's operation will do well.