The deferred collection option was rolled out last November, here is the announcement inc. screenshots.
http://community.quickfile.co.uk/t/new-options-to-defer-gocardless-collections/5216
I’m afraid there is no test environment for our GoCardless integration, you can however post nominal amounts as low as £1.00 to see how things work. You can then go into GC and cancel the payments after.