Khan Academy
Especially good for math 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
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).

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.

Mystery Science
Free science lessons

The Kids Should See This
Wide range of cool educational videos

Crest Awards
Science awards you can complete from home

STEM Simulators

Learn Computer Science