As mentioned above, there are other stair types, which do comply with the Building Regulations and these are really to be used when you're tight for space. And... from a practical point of view, moving furniture up and down those types of staircases will obviously be a lot more difficult than a traditional staircase, so it's not "just" the legality issues you have to take into consideration.
You mentioned a spiral staircase, which has now been knocked on the head as it isn't really favoured. People do think a spiral staircase will not take up as much room as a traditional staircase. However, they're wrong. To have a compliant spiral staircase, you would be looking at a min. area of approx. 1500mm x 1500mm, which isn't that small.
As for stairs and the bulding regulations, there are a few guidelines you'd have to follow. Have a read through Part K...
http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADK_1998.pdf
Things like... pitch, rise/going, handrail, headroom, etc...
Those are the main requirements for traditional staircases. The regulations would be a little more relaxed if you were looking at installing an alternating tread staircase... which is also mentioned in Part K.
I'm assuming you have taken onboard and complied with the other regulations associated with a loft conversion?
