Received: 4 May 2020
Accepted: 5 January 2021
First Online: 12 January 2021
Ethics approval and consent to participate
: This work was an evidence synthesis, so ethics approval and consent to participate are not applicable.
: This work was an evidence synthesis, so consent for publication is not applicable.
: MSB is a member of the Labour Party (United Kingdom). MT is a member of the Scottish National Party. The work was not conducted on behalf of or endorsed by any political party or organisation. No other authors declare any potential conflicts of interest.