The certificate can be used by any UK resident to prove where they are tax resident. As it is provided by HMRC, it has credibility as they will check the applicant's tax record before issuing it.As you will see here, you need to register for VAT if your turnover in the previous 12 months has exceeded £81,000 or you expect it to in the next 30 days.
If you do have to register for VAT and the Italian company you are dealing with is registered for VAT, you will not need to charge them VAT as they and you are within the European Union or the European Economic Area. They will, however, need to give you their VAT registration number and you should check that it is genuine using the facility here. If the Italian business is not registered for VAT and the services you supply are VATable, then you will need to charge them VAT.Normally, you only take account of the value of goods and services supplied within the UK when calculating your turnover for VAT registration purposes unless those goods and services are exempt from VAT.