ICO haven't yet released aggregate figures for GDPR, it's too soon. GDPR is a minor update of DPA, and they have released aggregate numbers on that for a while. Fines are levied in a tiny minority of cases. Warnings are far more common, as is steady escalation. The expectation here is the proportions will remain the same under GDPR.
On weed, actually no, because the default action for weed for the vast majority is just a warning. So no, it isn't clear that getting fined without warning first happens quite frequently, because that's also simply not true. You're very unlikely to see a court without a warning first.