Links in emails and texts are useful for users, but they can also pose a phishing risk.
To include linksyou’ll have to show that the level of risk is appropriate. For example, if you send a tax reminder and include a link to where the user can pay tax, that’s useful. If the only risk is that someone else pays that user’s tax, that’s probably an appropriate level of risk.
Links should only be to pages on GOV.UK, preferably the transaction start page.