Yes, they are all the same, but as sunmagic says there will be a difference between Spain and GB because of the exchange rates but they should be more or less the same. The things they sell may also vary slightly.
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Resist the fast fashion urge stateside for a while and stock up while you're in Spain. Not only is it incredibly cheaper (see above) but the assortment has much more depth and interest.
Zara and it's sister brands from the Inditex group (Bershka, Stradivarius, Pull & Bear) are of course very popular and somewhat cheaper in Spain than in other countries. They are also present all over Spain even in medium-sized towns.