Well for one, your i5-3570k is a third generation i5. It's not first gen. An easy way of telling this is by the first digit after the i5. The first generation i5's didn't have 4 digits, so it just had names like i5-650. 2nd generation then started adding another digit to the beginning of its model name to specify that it was a 2nd generation sandy bridge (e.g. i5-2500), followed by 3rd generation ivy bridge (i5-3570) and now 4th generation haswell (i5-4670).
2nd, the differences are in performance. Every generation has a new batch of processors with newer, (usually) more efficient architecture for better performance. However, Intel doesn't really push the envelope from one generation to the next, because they only have one source of competition (AMD) and AMD isn't exactly giving Intel a run for their money. As such, Intel doesn't need to put in that much effort into R&D to compete with AMD so there's not exactly a disparity in performance from a 3rd generation to a 4th generation i5.
Anyway, the i5-3570k is a fantastic processor. It's only a year old, but it's amazing at what it does. It'll easily net you top performance for years to come, whether you're gaming, editing, or doing whatever, and 80 quid is a fantastic price to get it at.