Fairly succinct response there (thanks) Treewind.
One more thing to consider for 'voluntary registration' (Registering when trading below the registration threshold)
If most of your 'customers' are VAT registered businesses then they can usually reclaim the VAT you charge them, so becoming VAT registered lets you reclaim VAT on your costs without (effectively) increasing the costs to your customers.
If your customers are non-vat-registered consumers then registering puts your price to your customers up by 20% making you less competitive.
Whether to register voluntarily often then comes down to the status of your customers.