Currently, I am studying A-level: maths, physics, computer science, geography and taking on an EPQ with dyslexia - after achieving 9,9,9,8,8,8,7,6,6 in my GCSES. When I was picking my options I did consider taking A-level DT as it was my highest grade 9, and I also wanted to go into mechanical engineering or geographical science/resource management at university.
Personally, I received a high 8 in maths and I do understand why some students do suggest the need for an 8 or 9 to even consider taking the subject. Yet from my understanding, the value of achieving a grade 7-9 in maths is purely so the student has already been introduced to some of the first year pure content to ease them in (the latter part of the GCSE spec some lower sets ignore). Therefore I think as long as you can meet this threshold (7) and have been introduced at least it would make their life easier. Yet you have to understand they will re-teach this content regardless so it isn't a drastic concern at the end of the day.
Physics is hard nonetheless - yet it's more about the application of the content to the question. I did combined science at GCSE with a dysfunctional science department, to say the least. I ended up getting a 7/8 in the final exams but this was through a lot of cramming and the support of my parents who had degrees in the subject (which clearly was a massive help compared to the peers in the class which I was eternally grateful for). Yet science teaching nationally can be varied at most which leave a lot of students underachieving. Though I see a lesser need to have done as well in the subject to take it - on the condition you work hard. However, I would suggest taking maths with physics to make your life easier.
From the research I have done talking to university representatives and looking at university websites, you need both maths and physics for mechanical engineering. If not your significantly behind on the maths, or significantly behind on the content. Mechanics is also in both maths and physics to different levels and prospectives so it is worth doing both especially if you want to go into mechanics.
Though if you wanted to go into the aesthetics of DT then physics wouldn't be needed. Yet maths would still be vital for the course.
My recommendation personally would be to speak to their DT teacher about the possible destinations of there past students, and the careers advisor ( if their school has one because I full well know my old school didn't until 9 months before I left.)