This is a complex area of tax law.
This reference suggests that:
I would suggest that this does not extend to areas where the customer purchases a voucher as such - it artificially reduces the price of the goods thereby reducing the collection of Value Added Tax due to HMRC rather than would be the case in the event of a price reduction or sale.
In my opinion, these electronic tokens generate a VAT point at the time of purchase by a customer and should be accounted for as tax collected on your VAT return. That will account for the reduced sale price (and thus reduced VAT collected) of the goods for which the voucher is ultimately exchanged.
The final arbiter (as always) is HMRC who should be consulted - I would say, in writing, just to be sure that any control visit later does not lead to a misinterpretion of your VAT records and you can be assured of the position.