HMRC do not give any allowances to the self employed at all. The self employed are treated exactly the same as the employed in that they get a personal allowance which they can earn up to before paying any tax. It's an allowance given to all UK taxpayers, nothing to do with being Self Employed particularly.
And no, if your profits are less than the allowance you don't get anything back. It isn't a sum of cash, it's just an amount you can earn before they will assess you for tax.
If however your profits have dropped considerably from last year it's likely you will get a tax repayment as you'll have made payments on account already to pay off this year's estimated tax liability and those will be refunded.