Well. Maybe my words “should” and “must” are incorrect then. I know for sure I was informed though (by TWO brokers at the time i.e. SB in the UK and CFD in Bulgaria). But now that you mention this: could be that the notifications were applicable to NEWLY opened positions initiated on or after a certain date and not existing positions. For the sake of interest let me see if I can find those emails (they’re on a backup).
To add though and pertinent to the actual thread topic:
The answer is YES they can I guess (whether it be increasing margin requirements i.e. decreasing leverage OR widening the variable spread).
Although in just looking again at the above:
I’m pretty sure it would cause a veritable shitstorm should the increasing of margin requirements result in a margin call for the client. Sure worth some investigation.
But I just saw the last point in your post Tom:
The BREXIT example used was for sure a foreseeable event i.e. not simply a news data event that surprised type of thing. So advance notice was indeed practicable.