As someone who has gone through the education system gcse a levels and has just graduated this summer from university. The best advice I could give you is as long as its not general studies it doesnt matter what a levels you do. Pick the a levels that you will perform best in, unless you want to study a degree in uni that REQUIRES certain a levels (e.g medicine requires biology and chemistry at most universities). All unis care about is taking on students that can work hard and produce good results. If do art, music, english and get 3A* its alot better than doing Maths, F.maths, physics and getting 3Cs. dont risk it unless your 100% sure you'll get good grades A*s.
Alot of grad schemes and summer internships look at a level grade alongside if not more than uni grades to wager which students are excellent. they dont care what subject it is, again they want talent that can work hard and produce good results. Every subject is useful in its own way, and gives unique skills recognisable in the working world. (e.g History = good research skills and report writing, Maths = analytical thinkers, Languages = paying attention to detail).
A levels is the last exams that all students in the country are on the same level playing field fundamentally, at uni its harder to fairly assess people because e.g Economics at cambridge might be more rigourous than econ at uni xyz therefore a 1st class from both uni may not equate.
They dont see it as she got a c in physics but the fact she did physics must mean shes smart to some level. No, the degree that dont need physics will see it as she doesnt peform to high standards. A the degree that requires it will say the grade is too low your understanding isnt good enough and wont accept you. so its lose lose. This is the mistake so many people make and end up retaking.
As for Btechs, unis and employers etc DONT CARE!!! They only care about ucas points!!! I know people reading at Durham, UCL, Nottingham that did Btechs.
Dont misunderstand me though, if you plan to study hisory as a degree its good to study it at level so you can get your brain into the swing of it and gage if its for you, like wise with engineering degree = physics and maths a level, but its not compulsory unless stated by the uni. You just need to display passion for it.and maybe what you plan to do with it in the future.
If in doubt call the university and ask.