What subscription is required to enable the fields on invoices to be included in emails/pdf's sent to clients

Which field do you mean?

https://quickbooks.grsm.io/UK

Hello Colin, 

Welcome to the Community page, 

If you have a look at this link https://quickbooks.intuit.com/uk/pricing/ it shows you all the different packages we have at Quickbooks and what features are available in each one and the price we are offering as well.