With the VAT retail schemes, you work out the value of your total taxable sales for any given period (one day for example) and calculate the VAT due on the total, rather than on each individual sale as under the standard VAT scheme.
You can only use the scheme for retail sales and VAT invoices must be issued to any VAT-registered customer who requests one.
There are several standard retail VAT schemes – point of sale, direct calculation and apportionment methods.
With the point of sale scheme, each transaction is identified as standard- rated, lower-rated or zero-rated at the time of sale – for instance with a till that will differentiate between goods sold at the different VAT rates. The VAT can then be calculated at each rate (perhaps the till will calculate these totals at the end of each day.
Under the apportionment 1 VAT retail scheme, sales are split between standard-rated, lower-rated or zero-rated in proportion to your goods purchased for sale. For example if 80% of your stock purchases are standard-rated then 80% of your sales for the same period will be treated as standard-rated.
For further details go to
http://www.hmrc.gov.uk/vat/start/schemes/retail.htm