€ Euros. The best place to exchange your money in Barcelona is at a bank as they have the most favourable rates, but be sure to shop around for the best deal. You may find it better to avoid the currency exchanges in the most popular tourist areas around the La Rambla and the Gothic Quarter as they have the worst exchange rates. ATMs are also good way of getting cash, but you should be careful how much your bank charges for the service. You can also exchange at hotels and in some shops.
The official language in Barcelona is Catalan, although some Spanish is spoken and it is largely understood. Most tourist hotspots have English speakers but don't expect people to speak any English away from the most popular parts. Most people are very patient especially if you have a few phrases in Catalan or Spanish to start off with.
+34 for Spain, then 93 for Barcelona followed by the local number.
UK citizens only need their valid passport to enter Spain.