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.

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This site is old and no longer updated and yet there's so much still available, from language learning to BBC Bitesize for revision.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.

Scratch
Creative computer programming

Ted Ed
All sorts of engaging educational videos

National Geographic Kids
Activities and quizzes for younger kids.

Duolingo
Learn languages for free. Web or app.

Mystery Science
Free science lessons

The Kids Should See This
Wide range of cool educational videos

Crash Course
You Tube videos on many subjects

Crash Course Kids
As above for a younger audience

Crest Awards
Science awards you can complete from home.

iDEA Awards
Digital enterprise award scheme you can complete online.

Paw Print Badges
Free challenge packs and other downloads. Many activities can be completed indoors. Badges cost but are optional.

Tinkercad
All kinds of making.

Prodigy Maths
Is in U.S. grades, but good for UK Primary age.

Cbeebies Radio
Listening activities for the younger ones.

Nature Detectives
A lot of these can be done in a garden, or if you can get to a remote forest location!

British Council
Resources for English language learning

Oxford Owl for Home
Lots of free resources for Primary age

Big History Project
Aimed at Secondary age. Multi disciplinary activities.

Times Tables Rockstar
An online Times Tables game. All children have a login.

Geography Games
Geography gaming!

Blue Peter Badges
If you have a stamp and a nearby post box.

Red Ted Art
Easy arts and crafts for little ones

The Imagination Tree
Creative art and craft activities for the very youngest.

Toy Theater
Educational online games

DK Find Out
Activities and quizzes

Twinkl
This is more for printouts, and usually at a fee, but they are offering a month of free access to parents in the event of school closures.