Consider becoming a volunteer responder. This is an additional service provided by the NHS. NHS Volunteer Responders is a new group that will carry out simple, non-medical tasks to support people in England who have been asked to shield themselves from coronavirus because of underlying health conditions. They will be used by healthcare professionals to make sure people who are highly vulnerable to coronavirus (COVID-19) are able to stay safe and well at home.

You can contact NHS Volunteer Responders at https://www.england.nhs.uk/2020/03/your-nhs-needs-you-nhs-call-for-volunteer-army/
N.B. Recruitment of volunteers was temporarily suspended on March 27th to allow for the initial wave of volunteers to be processed. Keep an eye out for recruitment to start again.

Or

More locally #TeamHerts Volunteering (#THV) is a countywide Hertfordshire service providing information for individuals who want to volunteer and for charities or similar organisations who are looking to recruit and manage volunteers.
(https://www.thvolunteering.org.uk/)

Or

Can you support your neighbours? There are many things you could consider offering, including, walking a dog, picking up a prescription, putting out the dustbin, taking in a parcel.

Talk to someone by phone to provide a bit of company. Or use Skype, Whatsapp or other video app for a face to face chat.

There are many other suggestions for volunteering on the internet.