The Download All feature works by moving a full series to your planner and then downloads the first two episodes automatically (making sure you have two available at all times). Watch Next, on the other hand, takes Netflix’s Post-Play feature and repurposes it for Sky+. It means that you can watch an entire season from start to finish by simply hitting the green button once every hour (think of it as Sky’s answer to the “Are you still watching?” prompt).
Source: Sky borrows from Netflix’s playbook with ‘Watch Next’ feature