Never been to Remington Mill specifically but the description you're giving - that heavy pressure feeling behind the eyes and the temperature drop that doesn't match ventilation - is something I've documented at three separate industrial sites in the north. Old mills are interesting because you've got decades of repetitive labour, accidents, probably deaths that never made it into any official record. That kind of embedded residual energy is genuinely different from what you get in domestic haunts.
What kit were you running when you visited? Because if you weren't capturing baseline EMF readings before anything happened then its hard to know what you're actually dealing with. I'd want to see at least 20-30 minutes of baseline before drawing any conclusions about anomalous spikes.
Anyone else with Coventry experience please chime in, the more data points the better. I'd potentially consider making the trip down if there's enough corroborating accounts from people who went in without knowing what to expect beforehand.