It never hurts to get a paper back, or even a remark if a candidate has a U, but expected much more, with realistic rather than deluded expectations. The only danger with remarks is that the mark goes down, but if there is already a U that is not going to matter.
In my experience, teachers usually know when a grade is likely to be an anomaly. When this is the case, we have seen a large percentage of remarks result in higher marks and grades. Occasionally there is a HUGE error and makes can go from a U to a B or A, but this level of error is very unusual. The remarks we usually get are for candidates who should have got a* or a and got the grade below.
Bearing in mind you say lots got Us, it could be a rogue marker or it could be poor teaching. Or of course,the candidates may not be as strong as they were led to believe. And yes, quite a few who were good at GCSE don't make the jump to A level well, although to get a U from a GCSE of A* seems unusual, so worth pursuing.
In my school, on results day,mew have people to advise from each subject and pupils apply for remarks or scripts back there and then, so that we often have them back by the start of term. We look at the individual marks received and advise on whether there is a real risk of a remark causing a grade to drop (most likely if the candidate is near a grade boundary) and also advise, based on the Uni they wish to apply to, the grades needed there and the overall profile of their results. I think that this kind of immediate advice is very valuable.....far too many people seem to receive no advice about what to do next. in Sept, we will be advising about resits which might need to be put in for, ensuring candidates don't do too many, as this is to the detriment of A2 and again, looking at the whole picture of their university of choice and how likely a higher grade is given their abilities. Anyone resisting will be required to have their script back, and to go through it with their teacher so they understand their weaknesses.
Incidentally, we only allow people to progress to A2 if they have a C or above at AS. Few don't have this, but can drop the subject as they usually go down to 3 AS. If they have several grades below a C, there is an option to reduce to 2 subjects, to retake the year or to leave. Pupils do not begin an AS unless they have an A at GCSE in that subject,mor a related subject if it is a new subject, so most are capable of getting Bs or above at A Level.