Are there any cases from the UK, whether from the Supreme Court or any other courts, saying that the COVID-19 pandemic can be considered as a mitigating factor for sentencing?
R v Manning includes the following (my emphasis)
We would mention one other factor of relevance. We are hearing this Reference at the end of April 2020, when the nation remains in lock-down as a result of the Covid-19 emergency. The impact of that emergency on prisons is well-known...
The current conditions in prisons represent a factor which can properly be taken into account in deciding whether to suspend a sentence. In accordance with established principles, any court will take into account the likely impact of a custodial sentence upon an offender and, where appropriate, upon others as well. Judges and magistrates can, therefore, and in our judgment should, keep in mind that the impact of a custodial sentence is likely to be heavier during the current emergency than it would otherwise be. Those in custody are, for example, confined to their cells for much longer periods than would otherwise be the case – currently, 23 hours a day. They are unable to receive visits. Both they and their families are likely to be anxious about the risk of the transmission of Covid-19.