Health Secretary Matt Hancock has said that those in self-isolation on medical advice should be treated as on sick leave and may be eligible for statutory sickpay. Thanks to Emma Ahmed of Hill Dickinson for doing excellent research and clarifying the position.
Section 151(4) of the Social Security Contributions and Benefits Act 1992 states:-
"… a day shall not be treated as a day of incapacity for work in relation to any contract of service unless on that day the employee concerned is, or is deemed in accordance with regulations to be, incapable by reason of some specific disease or bodily or mental disablement of doing work which he can reasonably be expected to do under that contract." It is most unlikely that self isolation is a “specific disease or bodily or mental disablement of doing work”, so no entitlement.
HOWEVER, regulation 2 of the Statutory Sick Pay (General) Regulations 1982 provides:-
(1) A person who is not incapable of work of which he can reasonably be expected to do under a particular contract of service may be deemed to be incapable of work of such a kind by reason of some specific disease or bodily or mental disablement for any day on which either: he is excluded or abstains from work, or from work of such a kind, pursuant to a request or notice in writing lawfully made under an enactment; OR by reason of it being known or reasonably suspected that he is infected or contaminated by, or has been in contact with a case of, a relevant infection or contamination.
“relevant infection or contamination” means... (i) any incidence or spread of infection or contamination for the purpose of preventing, protecting against, controlling or providing a public health response to, such incidence or spread.
The Health Protection (Coronavirus) Regulations 2020 contain a declaration by the Secretary of State that the incidence or transmission of novel Coronavirus constitutes a serious and imminent threat to public health, and the measures outlined in these regulations are considered as an effective means of delaying or preventing further transmission of the virus.
So assuming that someone who self isolates does so because they are given a written notice, typically issued by a GP or by 111, then they are deemed to be incapable of work, and so are entitled to statutory sickpay.
But if somebody chooses to self-isolate, and/or is not given that written notice, then they are not entitled to statutory sickpay.