A rolling period is not fixed like the tax year it is any 12 month period (August to July/February to January etc)
You do not have to have a turnover in excess of the VAT threshold to register for VAT as anyone can apply on a voluntary basis.
VAT Registration
You can register online yourself, or ask an agent to register for you. You will get your VAT registration certificate through the post within 30 days, you will also be able to view through your online account
You cannot charge or show VAT on your invoices until you get your VAT number. However, you’ll still have to pay the VAT to HMRC for this period.
You should increase your prices to allow for this and tell your customers why. Once you’ve got your VAT number you can then reissue the invoices showing the VAT.
Purchases & Services before registration
Once you have registered for VAT, either voluntarily or because you have exceeded the VAT threshold, HMRC will allow you to reclaim VAT on
4 years’ worth of goods if your still have them
6 months of services
You should reclaim them on your first VAT Return
After Registration
Once registered and have received your VAT Number you must put your VAT number on your invoices, and check the VAT rates for the goods or services you supply