If you require a licence and don't apply, you could face a criminal conviction upon prosecution and an unlimited fine or a civil penalty of up to £30,000. This may also lead to you being placed on the Mayor of London's public Rogue Landlords and Agents database.
You could also be subject to a rent repayment order and may have to repay up to 12 months rental income.
If a property should be licensed but isn't, the landlord cannot serve notice to quit on a tenant (section 21 notice) or seek possession of their property until the licence has been obtained.