Khan Academy https://www.khanacademy.orgEspecially good for maths and computing for all ages but other subjects at Secondary level. Note this uses the U.S. grade system but it's mostly common material.
Futurelearn https://www.futurelearn.comFree to access 100s of courses, only pay to upgrade if you need a certificate in your name (own account from age 14+ but younger learners can use a parent account).
Seneca https://www.senecalearning.comFor those revising at GCSE or A level. Tons of free revision content. Paid access to higher level material.
Openlearn https://www.open.edu/openlearn/Free taster courses aimed at those considering Open University but everyone can access it. Adult level, but some e.g. nature and environment courses could well be of interest to young people.
iDEA Awards https://idea.org.ukDigital enterprise award scheme you can complete online.
Paw Print Badges https://www.pawprintbadges.co.ukFree challenge packs and other downloads. Many activities can be completed indoors. Badges cost but are optional.