The title of the Statutory Instrument, and the year, are always written in italics in both your citation and your reference
The Ionising Radiation (Medical Exposure) (Amendment) Regulations 2018 state that....
The Ionising Radiation (Medical Exposure) (Amendment) Regulations 2018 (SI2018/121). Available at: https://www.legislation.gov.uk/uksi/2018/121made (Accessed: 4 April 2023)
When you view a Statutory Instrument using the government website, the full title, year and number of the Statutory Instrument will be shown, for example the year 2018, and the number 121 in the image below.
Write the title exactly as it is shown on the government website, including the brackets.