Where goods were bought by the business in the four years prior to the date of VAT registration and those goods are still on hand at registration, input tax may be claimed to the extent they are for use in making taxable supplies. This means that if the business uses those goods for exempt, non-business or private use, there would need to be an apportionment to reflect that. However, there is no adjustment required to reflect use prior to VAT registration. HMRC did at one point implement a policy of requiring such an apportionment and amended their internal guidance to this effect, but this was later rescinded. Assuming wholly business use in a fully taxable business the input tax would be claimable per the original VAT invoice.
However, before rushing to register there are a couple of other things to consider.
Firstly, who are your customers? If you supply mainly individuals or unregistered businesses, you may find it difficult commercially to pass on VAT on your sales in full. If you are unable or unwilling to increase your prices and you have to account for output tax out of your turnover, effectively paying 1/6 to HMRC, does the VAT recoverable on the purchases outweigh that additional cost to your business? If not it may be better to defer voluntary registration until closer to the four-year time limit for claiming pre-registration input tax, or even sacrifice it altogether. There is less scope for delaying registration while still being entitled to input tax where the VAT has been incurred on services, as the time limit for pre-registration input tax on services is only six months.
If your customers are mainly VAT registered businesses in a position to recover the VAT you charge, then registering voluntarily should not create a problem, other than the additional admin.
The other issue to check is the entity. A problem arises where the entity that bought the goods is not the same entity that is now registering for VAT. Usually, this is when a sole proprietor starts the business but doesn’t register for VAT and later incorporates, and only the company VAT registers.
Even if the purchases were made within the time limits, by a person who became an officer or employee of the business and who has been reimbursed, if they were not bought specifically for the purposes of the company it cannot recover the VAT. Where goods or services have been bought for the SP business, the only entity with any entitlement to input tax would be the SP.