Unfortunately government guidelines for employers are pretty relaxed. They say employers should make every effort to ensure social distancing is maintained. For working in customer homes, I would have thought this should ensure checking in advance whether anyone in the household is or has within the last 7 days displayed symptoms of temperature or cough, asking customers to provide you access to the nearest door and a sink for handwashing, to ensure everyone stays as far as possible away from you while you are working. And providing you with gloves, wipes and alcohol hand cleanser.
If your employer is making reasonable efforts, they don’t have to pay you if you decide not to work. The exception is if you are self-isolating for up to 2 weeks due to you or someone in your household having symptoms of COVID 19, in which case they must pay you SSP (around £95 per week), which they can claim back from the government.