Why is London milder than Newfoundland at the same latitude?

Heat carried north by the ocean. The Atlantic circulation moves warm surface water towards the northeast Atlantic, where it gives up heat to the air before sinking - and the prevailing westerlies carry that warmed air over Europe. St John's and London are at similar latitudes and have completely different winters.

The Gulf Stream gets the credit and deserves some of it. The fuller explanation includes the prevailing wind direction, and the fact that western coasts of continents at these latitudes are mild anyway - the Rockies also block and reshape the flow that reaches eastern North America. Attributing all of Europe's mildness to ocean heat is a simplification that oceanographers have spent decades qualifying.

The concern is what happens if the circulation weakens. Fresh water from Greenland dilutes the salty surface water that drives the sinking, and measurements suggest the system has slowed. Estimates of when it might collapse range from this century to not at all, and the consequence would be regional cooling and disrupted rainfall rather than the abrupt global ice age of the films - severe, and not the same thing.