If you go on line and search "quiet time Morrison's " plus your branch location, it may well come up with a chart showing when it's busy, I've used that for BQ and Tescos, and just checked it worked for Morrison's GY.
We went into GY early Monday, the wearing of masks in most stores was surprisingly good, only one customer not wearing a mask, however towards the end of the visit, all the grokles started appearing and it was getting crowded on the streets, so we then started wearing the masks outside as well.
The place to avoid were the market food / coffee stands, neither staff nor customers wearing masks, many customers leaning on counters and definitely not distancing..
Officials want you to wear masks to not infect others, however if you are breathing in, through a mask, any airborne particle must stand a chance of being captured by the cloth, especially since the virus is carried by the damp breath of the exhaler.
We use the so called disposable masks, however they are disposable because that's what the NHS do as its cheaper than washing them. We have several each they are stored on the car dashboard between uses. By the time they are needed again for the next weeks shopping the will be clear of any virus. After a couple of uses they get thrown in with the wash. They won't last many washes but it extends their life.