Impulse buying and taking the word of a store salesman is a recipe for stress.
My advice to other people would be this: research first *very* thoroughly. Seeing as it was a £579 iPad and not a portion of cod & chips, and seeing as it has cost you all that time and stress, then I am bemused that you simply strolled into the store, asked some sales assistant "is it unlocked?", they say "yeah, tis" and you handed over nearly 600 smackers?
I sympathise, having skimmed over this story, but I would also say you are also responsible for not having done your research, looking at the considerable cost of the item.
All the best, God bless you 🙂