Wasn't at Lemp in October but I've done a fair bit of basement investigation work over here in Suffolk, and what you're describing rings familiar - that sudden drop in ambient temperature combined with the pressure sensation behind the ears?
Worth asking: did anyone have a K-II meter or an Ovilus running at the time? I'd be curious whether there were any EMF spikes correlating with whatever you felt. In my experience, those physical sensations rarely happen in isolation from instrument activity, and the timing usually tells you something useful.
The Lemp basement specifically interests me because the building sits on what appears to be a fairly significant underground water source - the old brewery infrastructure. Flowing water beneath a site can amplify certain phenomena considerably, and it's something I track closely given my interest in ley lines and earth energies. There's a reasonable argument that geological features contribute to why certain locations generate consistent, repeatable experiences across multiple visitors.
A few questions for those who were there:
Which part of the basement - the tunnel section or the main cellar area?, Was it a gradual build-up or sudden onset?, Did it affect everyone in the group simultaneously or just individuals?
The collective vs. individual aspect matters quite a bit for ruling out suggestion and environmental causes. Would genuinely like to compare notes if anyone logged timestamps on their equipment that night.