Ofqual rules are very clear that awarding organisations must ensure they do not disadvantage any learners.
Therefore, any work completed at home after the closure would only be accepted under the say so of the exam board - learners who do not have access to resources when others do, those who have been ill or had family bereavement due to covid, support from those around them, young carers etc. All of these may be impacted and be unable to complete tasks to the same level that they could have done had school been open. The actual subject will be impact this also - they need to evidence computer programming without the software at home would be a disadvantage therefore home activity not accepted, I would expect areas such as English be more accepting as it is less resources orientated.
The work also needs to be authenticated as learners own work which can be a challenge for some subject areas.
At present schools do not know what evidence will be required to submit for grades (ofqual haven't released this yet).
If you have a child who is border line grade - I would encourage them to create evidence of the higher grade, update older stuff etc - might not be usable but on the off chance it can be it is worth doing.
Look at what their progression is and start to look at building up enhanced knowledge and portfolios in that area - look at open uni activities, wider reading, doing creative activities, building up vocab to prepare for sixth form/uni. If they are going into workplace, look at employability and subject knowledge around this.