I would have thought that the main reason for doing some sort of exam fairly immediately on return is so that staff know what needs to be done. You don't want them just to teach what they normally teach in year 13 as if nothing has happened, do you? But you also don't want them to spend all term doing last term's work again, and still be finishing the syllabus in May.
If they give them an exam, they can then see what needs to be revisited with the whole class, what might need some extra sessions for some pupils, and what they've all got the hang of and doesn't need to be worried about. The sooner the better, so they can work out how to fit it all in.
They will need to do predicted grades very soon for those applying for Oxbridge/medicine/vet/dentistry. Hopefully those kids will be reasonably good independent learners, and so they'll be able to show what they can do. The rest of the UCAS applications have to be in by January, so hopefully as well as these exams, there will be something towards the end of term when they are more up to speed.
Make sure your kids know that the most important thing is that, if they are not on top of things, the teachers know so that they can do something about it. Far worse to have exams in November and it get discovered then, too late to rectify anything before UCAS.