From the Motability Coronavirus info page:
I am due to collect my new vehicle, but I am self-isolating, what should I do?
In these exceptional circumstances, if you are unable to attend your dealership to collect your new vehicle you can give consent to a family member or friend to collect the car and enter the PIN on your behalf.
The family member or friend must be a named driver so they are insured to drive the car away. If you need to add them as a named driver please contact your dealership prior to the collection date to arrange this.
When collecting the car they will need to take your PIN letter with them to the dealership, if they do not have the PIN letter then the dealer will call you to get your consent.
Alternatively, you can contact the supplying dealer to rearrange your collection. They will be able to move your collection date which will automatically extend your current lease.
Updates from the Motability Scheme regarding COVID-19 (Coronavirus)