I think you’ll find it almost impossible to get in without a very very good reason.
I’ve got some pretty major DV clearance that’s allowed my escorted visits to places like Sellafield and the AWE. This clearance outstrips my everyday warrant
I can imagine and rightly so; wouldn't want anybody just turning up at such places. Someone told me once how much their security clearance cost their firm in monetary terms alone to get, surprising how much it costs.