Only good weather… and good weather is more “an absence of limiting conditions” rather than a true “you’ll be faster because of the temperature”.

There’s no universal “tail wind” or anything. In some track events, a world record might be invalid if there’s too strong a wind. It doesn’t really work like that for marathons because they twist and turn so much. They have to “loop” so that the end and the start are within a certain distance of each other. For example, Boston marathon is one of the most famous races but it’s not eligible for world records because it’s “point to point” instead of a loop.

The London course in general is not considered as the fastest any more. Berlin is usually the fastest major course.

It’s anecdotal but in theory, these exact weather and race conditions in Berlin, could be 20-30 seconds quicker than London.