I think one way to resolve this is for the exam centres to allow candidates to sit a mock paper in exam conditions
Maybe home ed groups could organise a few parents to club together for the invigilator?
What part of the current rules about non-essential travel, schools only being for keyworkers and no gathering are you not understanding? There is NO WAY that kids, adults or whoever will be sitting GCSEs, mocks, any other sort of formal assessment this year. Not happening.
Yes it's unfortunate for children who have opted out of school and were preparing at home. But this is a tiny percentage of the GCSE/A-level/Scottish Higher candidates. September isn't ideal, but let's be honest, it's the best option there is right now.