A few years back I was in Porth yr Ogof when they were filming a scene for Britannia. Lots of extra fibre-glass rocks brought in, lighting rigs, etc. At one point I thought I heard a flood pulse coming and had a bit of a panic until a giant camera drone came sweeping in and landed on a platform they'd set up just short of White Horse Pool.
Nice thing was they kept letting me take groups through while they set up. Then when it came to filming they let me know where I could still go but not the entrance chamber. So we lay with our torches off in a side passage and watched.