Welcome to Flyertalk and welcome to the BA Forum.
I've seen various times mentioned as the cut-off, but it seems more of a guideline. There are times of the year and days of the week when Heathrow asks for the Early Bag Store not to get too congested, more of a T3 factor. At T5 I would say it's highly likely that it would be OK, unless it was one of those peak times.