Mate, you're not losing it. Shadow figures are one of the most consistently reported phenomena across locations like that - old industrial sites especially seem to attract them.
I've been doing paranormal photography around Essex for a couple of years now and I've caught what looks like shadow movement on camera more than once. The tricky bit is always ruling out the mundane stuff first - light bleeding through broken windows, other people on site, that sort of thing.
Few questions that might help narrow it down:
Did it have a defined shape or was it more of a shifting mass?, Any sense of it being aware of you, like it reacted to your movement?, What time of day/night was this?
From a demonology angle, persistent shadow entities at old industrial sites are interesting because of the residual emotional energy - mills had pretty grim histories in a lot of cases. Whether that's a residual haunting or something more intelligent is worth figuring out.
If you're planning to go back, genuinely recommend taking a second person and some kind of documentation setup. Even a basic trail cam helps massively for corroborating what you saw. I use a Reolink for static coverage when I'm moving around a site.
Would love to hear more detail about exactly what you saw.