I get my Morrisons shopping delivered every Saturday and to ensure a decent pick of time slots, I order the minimum on a Wednesday and then just edit the order the day before delivery.
I've just received an email from Morrisons advising me that 'due to a much higher than usual demand', they're running out of delivery slots fast. So I went to book and got one of the last available Saturday slots, even though today's only Monday 
I suspect it may be Coronavirus related panic buying (although I suppose they might just be short staffed). Either way you might all want to check availability a bit earlier than you normally would.