Illuminati-R-Us

.... who could take a stab at answering this?

British Rail. You CANNOT invade a country in railcoaches like that, and the German ICE has only been certified for channel tunnel use recently.

Maybe the personality of Adolf Hitler played a role, too. He was buried in a trench in WW I, and he had a deep-seated fear of underground locations.

Lastly, the tunnel is between Britain and France, not between Britain and Germany.

Is that helpful?

Yes, all perfectly good reasons.

I think my learned friend has covered all the salient points in a comprehensive and concise manner.

Unbeknownst to all of us, their time machine was down for regularly scheduled maintenance at this particular point in history?

Because it was too well guarded. I mean, duh. The same reason the allies didn't use it to invade France.

Wasn't it too small then too?

It was never reported in the press, but once they laid down the mulberry harbour floats, they were embarrassed to find the trucks riding atop the floats were no longer able to fit through the tunnels.