As the others have said, its not the end of the world if you cant do further maths. I didnt take my gcses cos of covid and Ill be sitting my a levels next week. I have an offer to study Aerospace at QM and I took physics, maths and cs for a levels. Honestly fmaths is only required for the top top unis like camb and imp and even then ik some guys whove been offered a place to study at these unis without doing fmaths. If it all goes bad, you can still go to an average russel group to study eng without further maths. Dw about it! You will do great.
As for exam techs, just make sure u read the question carefully and check the units. If you dont know how to answer a particular question, look at the context, imagine it check the num of marks try and fit it into a topic that you know and just reverse engineer the steps. cos ik with some questions u just dont even reognise the topic on which the question is being tested. 89/100 is truly incredible! so dw.