I stay at the DT Islington a bit and the room choice the day before are almost always based on your booked room type and are very limited.

About 50% of the time I am upgraded late on the evening prior, and the remaining 50% of the time early afternoon on the day of arrival. In both cases the room selection opens right up and gives a lot of choice.

If you are not arriving until the afternoon, I would play a waiting game. I certainly never check in early online here since I wised up to the timings for the inventory and upgrades to be manually processed.