Hi,
You are only able to recover VAT as input tax if you are VAT registered and you are only able to recover input tax prior to your date of VAT registration if you have purchased goods in the last 4 years and you were still in possession of those goods at the date of your VAT registration.
If you are able to successfuly backdate your VAT registration then you can treat the VAT incurred from that point if you meet the conditions of input tax as below:
VAT guide (VAT Notice 700)
Thank you.