It is never a good idea to proceed with a loft conversion when you have not gotten building regulations approval. In the same vein, not complying with building regulations can expose you to a lot of legal problems. For one thing, your local building council can alter or tear down the work if they judge it to be below the approved standard.
If you insist on proceeding with the alterations and consistently fail to meet with the approved standards, it can lead to legal action that may result into a fine. And in the event of a fatal accident that occurs as a result of non compliance with building regulations, you stand the risk of facing criminal charges.