Hi, OP -
I am a former Russell Group STEM admissions tutor (apologies to those who ate bored of reading this).
I think your DS needs an Insurance offer lower than his Firm offer. Here are some reasons why.
First, Lancaster is a wonderful option. The university, which is part of the RG+, and the School of CS are both excellent. DS’ offer of BBC is heavily contextualised, down from the standard offer of AAB.
Second, last year Overseas students turned against the UK as the new visa regulations were perceived as hostile (even though they didn’t affect many undergraduates). This year, Donald Trump is generating much greater hostility, particularly with China, even as we write. Overseas students can still apply through UCAS, so the UK may get a late surge of Overseas students. This would raise standards for Home students.
Third, many Schools will accept more slippage in students who have Firmed them than in students who have Insured them. When my School was becoming more elite we sometimes had to take students who did not meet their offer and we gave preference to students who had Firmed us. We would only look at slippage in students who had Insured us to keep from going into Clearing, which was pretty rare.
If DS is willing to go into Clearing or risk a gap year, that makes the decision easier. Personally I think Lancaster is a fantastic option.
Also, as DS has qualified for contextual offers at Leicester and Lancaster, it is possible that Sheffield has missed his contextual flags. If he wishes to investigate this, it would fine for him to send the admissions team a very polite email saying that Sheffield is his top choice, however (some or all of) his other offers are contextual. Thus he wonders whether he might be eligible for a contextual offer at Sheffield? He understands that different universities may use different criteria and thanks the admissions team for considering his question.
No one will take offence at this. The message can go to anyone on the admissions team; whoever gets it will route it properly. I am not saying he needs to do this but he is well within his rights if he wishes to.
Best wishes to DS.