The option to cancel your UK Work Visa and ask for a refund due to Coronavirus COVID-19 has now closed, you are not able to email the Coronavirus Immigration Helpdesk no longer (UKVI did not give any warning or date as to when this would happen, UKVI just went in and updated their 'cancel your visa' guidance recently).
However, if you have already attended your appointment, given your biometric data and received your approved visa and you want to cancel your visa, you can contact UKVI and ask to withdraw your application. Your visa fee is usually not refunded, but you can ask that your IHS (Immigration Health Surcharge) fee be refunded to you as you did not enter the UK and use the services of the NHS.
To contact UKVI for applicants outside the UK, go to this LINK.
See this LINK for more details on 'Cancel your visa'.