You would be served with an enforcement notice, requiring you to pay the amount due, and if you failed to comply with that you could be subject to fines of £11.04 a day for each worker involved. You could also be prosecuted in the criminal courts for a range of offences associated with the minimum wage, including a deliberate refusal to pay it, fiddling the records, or attempting to mislead HM Revenue and Customs (which is the investigating authority). If convicted of any of these offences you could be subject to a fine of up to £5,000 in respect of each offence.
There are plans to amend the penalties and strengthen the enforcement regime in the new Employment Simplification Bill.