The advice from all UK governments is that, wherever possible and safe, children should be kept home.
All nations of the UK have made provision for ‘vulnerable children’ to attend school where necessary. This includes children with significant additional support needs and higher level special educational needs, and children with ongoing social services involvement.
In England, the guidance on ‘vulnerable children’ who may be offered a place in school specifically includes adopted children and children on a special guardianship order: https://www.gov.uk/government/publications/coronavirus-covid-19-maintaining-educational-provision/guidance-for-schools-colleges-and-local-authorities-on-maintaining-educational-provision#vulnerable-children-and-young-people
Guidance in Scotland does not specifically mention adopted children, but gives local authorities, schools and health boards latitude in identifying children who may benefit from attending school or who need additional support.
In Northern Ireland, the guidance on vulnerable children specifically includes adopted and previously looked after children: https://www.education-ni.gov.uk/news/weir-sets-out-plans-education-provision
The definition of ‘vulnerable’ children in Wales does not specifically include previously looked after and adopted children. However, the list of included children is not intended to be exhaustive and schools are able to offer places to other children who are identified as ‘vulnerable’ or disadvantaged: https://gov.wales/guidance-supporting-vulnerable-and-disadvantaged-learners-html