Generally, you should aim to include in the UCAS reference all the information that you believe is required to assist the admissions decision maker in assessing your students' applications. Sending additional information in a separate email or letter, unless specifically requested by a university or college, frequently will only serve to complicate the application process, as the additional material may not be quickly or automatically matched with the original application materials.
There are however specific situations when a follow-up reference may be warranted:
Referees should ensure that when circumstances impact on a cohort of students, all of the universities and colleges concerned are contacted, and that as much detail is provided as possible to ensure that the new material is easy to match with the original application (eg full name, date of birth, Personal ID and degree course).