Well, it can be complicated (of course), presuming your client is VAT registered the safest thing to do is show a full VAT invoice all the time from your programing point of view (the rules is full invoice for £250 or over, simple invoice for £250 or under, but you can use full invoice from zero up )
below is extract from ukgov site : (the important bit, you need to know this regardless)
A VAT invoice must show:
an invoice number which is unique and follows on from the number of the previous invoice - if you spoil or cancel a serially numbered invoice, you must keep it to show to a VAT officer at your next VAT inspection
the seller's name or trading name, and address
the seller's VAT registration number
the invoice date
the time of supply (also known as tax point) if this is different from the invoice date - see below
the customer's name or trading name, and address
a description sufficient to identify the goods or services supplied to the customer
For each different type of item listed on the invoice, you must show:
the unit price or rate, excluding VAT
the quantity of goods or the extent of the services
the rate of VAT that applies to what's being sold
the total amount payable, excluding VAT
the rate of any cash discount
the total amount of VAT charged
If you issue a VAT invoice that includes zero-rated or exempt goods or services, you must:
show clearly that there is no VAT payable on those goods or services
show the total of those values separately
If you make retail sales and you make a sale of goods or services for £250 or less including VAT you can issue a simplified VAT invoice – see the section in this guide on simplified VAT invoices.
Now the delivery part !
Postage, packing and delivery within the UK included in the sales contract but charged for separately, eg mail order has the same rate as the goods being delivered or posted
Postage, packing and delivery within the UK charged as an optional extra is always standard-rated
So safest option is to charge and show this on the invoice as standard rate (20% at the moment)
ie.
My company
My Address
VAT Num : my vat num
Dated : todays date
Item Unit Price Quantity Total Price
(ex vat) (ex vat)
My Goods £10.00 2 £20.00
Delivery £10.00 1 £10.00
TOTAL ex vat £30.00
VAT @ 20% £6.00
TOTAL inc vat £36.00
That will cover all bases Presuming trading address is the same as registered address, if not, that needs to be there as well.