@Builder,
The other EU-members wanted the UK to become a member resp. wanted them to stay in the union - so the UK got all these concessions.
The "eurozone" is the monetary union of 19 of the 28 European Union member states which have adopted the Euro is their currency.
Denmark and the UK are left out due their 'opt-outs' from joining the euro: the UK didn't join the European Exchange Rate Mechanism already in 1979 but entered the ERM in October 1990, left it in 1992. (Denmark is still part of it: there's just one currency in the ERM II, the Danish krone.)
Actually, the Euro (or the common currency, before it was the Ecu) is dealt with in the
Treaty on the Functioning of the European Union with just one out of 24 titels (Articles 119 to 144 concern economic and monetary policy, including articles on the Euro, see
>here< )