What I have noticed is that it wasn't possible to get a single electric blanket with the flex on the left ...

I've never used one, so I wouldn't know but can't you just turn the blanket over?

But doesn't UK electricity only flow from Left to Right ?

That can't be correct. With the flex on the right, the electricity would be flowing out and your appliances would be powering the National Grid. I think it only flows from left to right in the southern hemisphere.

DC