BBC Learning – This site is old and no longer updated and yet there’s so much still available, from language learning to BBC Bitesize for revision. No TV licence required except for content on BBC iPlayer.
Futurelearn – Free 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).
Blockly – Learn computer programming skills – fun and free.
Khan Academy – Especially 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.
Seneca – For those revising at GCSE or A level. Tons of free revision content. Paid access to higher-level material.
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.