The amount of work required for a First compared to a 2:1 is often a huge increase. A 2:1 is the ideal degree class really - it puts you in the top 50% of graduates (so the top 25% of the population, at least academically and as a rough guide) but whilst it requires research and work on your part, you still have time to do things outside of work.
A 2:2 or Third is frankly worthless if you're hunting for a job.