The AMSP (Advanced Mathematics Support Programme) has developed resources to assist teachers and students during school and college closures. The resources focus on target groups of students to support their progression from Year 10 to Year 13 (ages 14-18) and beyond.

Note the menus for Students and Teachers at the top of the page which help you find appropriate resources. Choose the Student menu, then the appropriate option for the level of study.
For Year 10 enrichment activities are available. The student files are a series of slides which a teacher could present online or students could use independently. Teacher notes are available for each activity. The Collatz Conjecture slowed the writing of this post! A great activity, I like the Numberphile video from Alex Bellos which provides a wonderful introduction to the activity.
Whilst preparing our Year 10 students for further study looking at interesting mathematics like this, particularly in the current situation, will add variety to their work at home.
For Year 11 GCSE to AS and A level Mathematics transition resources will be so valuable. These resources on essential skills are really well structured, with a skills check, practise exercises with solutions, and a chance to explore some enrichment ideas. These materials should really help students prepare for A Level Mathematics.
See also MEI’s Support Resources page and schools can register with AMSP here.