You may be asked for payment at any point during a Course of Treatment (CoT). Some dental practices may ask for the whole payment up front, during the CoT, or after the CoT has finished.
NHS dentists can only charge patients under the rules set out by the Patient Charge Regulations. Practices cannot charge any additional fees above the NHS charge bands for providing NHS treatment. This would include taking a deposit before an initial assessment of your treatment. Deposits can be taken after the examination has been completed, however this should be deducted from the total charge band due.

Further information can also be found on the NHS Choices website.