We need to be careful here with terminology. If your "self employement" generates a trading loss then yes this would be offest agaisnt taxable income and if you have already suffered the tax via PAYE you would be due a refund once you have completed a self assesment return.
However, this would need to be a "trading loss" e.g. you have incurred more costs than income generated. If you have invested in stock, and still hold it, this is NOT a trading loss, as you still have the stock in hand.
Hope this helps!