Not checked it out yet, but it's on my radar. Probably going to be quite tight as they've just closed. Also M&S has just moved out of its church street location that's another one I'm keeping an eye on.
The old M&S will be quite tricky. M&S also still own that site. I used to work in the old M&S and I’m familiar with the whole building. There’s no chance you’ll get in on Ground level. Your best bet would be through the underground tunnel that takes you to the old delivery bay for M&S. if you can manage to somehow dodge or climb over the gate at the beginning of the tunnel, there’s a chance you might be able to try a little side door by the M&S delivery bay. But you have to go through the whole tunnel to reach the delivery bay. If that side door is locked, then there’s no chance.
But If you manage to get in, there are so many interesting parts of the building to explore. About half of the staff only areas in the old M&S were already abandoned for decades. The fourth and fifth floors have particularly been abandoned for decades, but I think the door to that area is locked.