Seriously? Well, a couple of the Australian banks did suspend trading in sterling over the weekend, since they had no idea what price sterling would be when the markets opened on Monday. Which had the result that ATM cards issued by these banks could not be used in UK ATMs to withdraw sterling (and UK-issued ATM cards could not be used in the banks’ Australian ATMs to withdraw AUD). I read reports of similar problems from Greece. Obviously this could inconvenience travellers.
SFAIK these temporary suspensions of sterling trading have now ceased, and cards work again. But I suppose if sterling continues to be wildly volatile the problem could recur.
Therefore, if travelling to sterling-land, it would be prudent to bring plenty of spare sterling, and not assume that you will be able to get it out of a hole in the wall.
Not if trading in sterling has been suspended, it can’t, which is the risk you are trying to avoid.
Admittedly, it’s unlikely that the British banks will suspend trading in sterling. But they could and would, if the markets were volatile enough, so if you’re seeking to avoid risk, then you don’t put yourself in a position where you need to buy sterling to meet your daily needs.
Sure, buy buying on Monday before you leave, you may miss an opportunity to buy more cheaply on Wednesday after you get there. But the opportunity cost of that may be a price you are willing to pay in order to have the reassurance that you will have the sterling that you need.