And you'd need to get some decent flooring in, so that gear/knees don;t get wrecked during dives, and you don;t end up slipping on broken paint.
And it probably will cost more, but indoor sites don't have to worry about the nasty english weather, and being placed in an industrial site will probably mean you can more people in through public transport
edit:: Just had a look online though, and WOAH. No way will there be one of a decent size that would be affordable.