Well, in general, all EU law is valid in all member states, this would include replacing their old currency with the Euro. However, it is possible for member states to negotiate "opt-outs" from certain treaties or legislation in the EU, which means that they basically don't participate in those fields.
At the time, the UK negotiated an opt-out from the Maastricht Treaty, which is the treaty in which the member states agreed to introduce the Euro as their new currency. (Denmark had also negotiated an opt-out, so they also kept their old currency).
The UK is considering the introduction of the Euro though, and they might introduce it in the future.. =)