C_J
It isn’t linked to whether Labour get into power. From a quick search:
There are 7 non-eurozone members: Bulgaria, Czech Republic, Denmark, Hungary, Poland, Romania, and Sweden who continue to use their own national currencies.
All but Denmark are obliged to join once they meet the euro convergence criteria, which is based on economic indicators (inflation, public debt and public deficit, exchange rate stability and the convergence of interest rates).