I'd like to imagine the forums are hosted in modern cloud infrastructure which can be dynamically scaled based on load. So if no ones using it at night it should be somewhat nominal in cost to run, then when loads of people are using it it gets automatically ramped up to cope.
Besides, OCUK make an absoluete fortune, they can afford to run forums 24/7 and not charge for them .
Pretty sure it's still run off a Core2duo machine in a beige box in the corner