Hi,
The figure you put in box 6 is the net value of your turnover for the period of your VAT return. If there is no VAT to be charged then the box 1 should show a nil figure and the box 6 the value of the supplies. You can see the guidance here:
How to fill in and submit your VAT Return (VAT Notice 700/12)
If you are not making taxable supplies in the UK then this will be a voluntary registration. However, you still need to complete VAT returns on a quarterly basis and include the figures for your supplies even though your output tax will be zero.
Thank you.