I see this in the middle of a two-way road.
If there are two parallel yellow lines, it is a no passing zone and you cannot cross over the line. The only slanted white lines I've ever seen were temporary dotted lines applied crookedly by construction workers after a road was resurfaced, but I'm sure that is not what you are talking about.
In the U.K., Rule 130 of the Highway Code states:
Areas of white diagonal stripes or chevrons painted on the road. These are to separate traffic lanes or to protect traffic turning right.
•If the area is bordered by a broken white line, you should not enter the area unless it is necessary and you can see that it is safe to do so.
•If the area is marked with chevrons and bordered by solid white lines you MUST NOT enter it except in an emergency.
I wholeheartedly agree with Pilsner Man's comment, too…
A thought shared.
It means Stay off this area.
It cannot be crossed. Hopefully when you see this, you are a passenger and don't have a license.