The RSGB site lists several organisations that provide online training for the UK amateur radio Foundation or Intermediate exams
Among those providing a free online training course at both the Foundation and Intermediate level is the popular GM6DX site.
Their Foundation course consists of 58 lessons which include PDFs, videos and quizzes. All material is written in a way that is suitable for all age groups. The course contains five mock exams where the questions are in a format that represents a realistic exam question. The Intermediate course comprises 86 lessons.
As the courses are online, material is available 24 hours a day, seven days a week. You can self-enrol at any time, see the website at
https://gm6dx.thinkific.com/
The amateur radio exams are carried in your own home, online with an RSGB invigilator present via remote access.
Details of other courses can be found at
https://rsgb.org/main/clubs-training/for-students/online-training-resources-for-students/
Online ham radio training resources
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