Contact your local care services or local charities (particularly those that support people with specific conditions) to see what sitting and befriending services are available near you or look at Befriending Networks which provides an online database of befriending services across the UK.
Specific services include your local Age UK, which also has a telephone befriending scheme, and Independent Age which offers regular phone calls or visits for older people. Both Scope and Mind also offer befriending schemes.