Can a tank attack across water within the same space (Germany to UK)?
Here's the snippet of map we're talking about:



There are three spaces of interest here:


England (a land zone)
The water surrounding England (a sea zone)
Western Europe (a land zone)


These are three separate spaces, and must be passed through in turn; Western Europe is not adjacent to England. You can't just jump from Western Europe to England, you need to pass through the sea zone on the way there. Unfortunately, tanks can't enter sea zones except by boarding a transport.

So getting a tank from Western Europe to England looks like this:


A German Tank starts in Western Europe. It can't enter the sea zone, because it's a tank.
Germany has a transport in the sea zone surrounding England, and there are no Allied sea units in that zone.
Tank boards transport, entering the sea zone in the only way it can.
Tank exits transport onto England, and gets involved in the combat there.