but web hosting isn't that expensive anymore, you can get unlimited web space and bandwidth now for like 20 pound a month, i no ocUK would be a bit more than this due to the fact that the servers they use will take more than 45 percent load due to the traffic (most web hosts disable accounts that exceed a 45 percent sever load). in which case they probably use dedicated servers, still from a web hosting company. a decent quad core server with 250gb hdd would cost around 150-200 pound per month, they probably have two dedicated servers. plus the forum is powered by Vbulletin the annual cost for this would be around 100 pound a year maybe, i use free forum software (SMF) so i don't really know about Vbulliten lol.
i could be totaly wrong but this is how i think they do it
Edit/ i mean 2 dedi servers for the whole of ocUK, not just the forum, becuase if you look in the domain its actualy a sub-domain (
www.sub-domain.website123.co.uk for example) so its just an extra lease of of the main domain site.