Can I use the Power Grid Expansion "Northern Europe / United Kingdom & Ireland"?
You can use the expansion with your existing copy of Power Grid,
with one caveat: the backs of the power plant cards have changed, which means you need to shuffle the deck while both the fronts, and backs are hidden when using the Northern Europe part of the expansion.

If you chose to use only the United Kingdom &amp; Ireland part of the expansion no adjustments are needed.