Choosing a degree program with 'low academic standards'
Since the sentiments already appear in your question, let me make this extremely clear:

Modern university degree programs serve at least two distinct purposes: education and certification

I entirely agree with you that a if you already know the material, for the education purpose of the degree it seems redundant to take a course on a subject you have already mastered. But as you have already indicated in your question, employers, graduate schools, and other agencies may choose to use your university transcript as evidence of said mastery. (Though, in some fields, the professional society provides their own certification function. For example, the society of actuaries have their own pathway toward membership. Those fields may be more "friendly" to someone without a formal degree.)

But I do wonder how you came to the assessment that you already know most of the material in offer for the degree programs. The Dunning-Kruger effect is a well-documented phenomenon in which practitioners of low expertise tend to overestimate their competency. While I am not saying that this necessarily apply to you, frequently in a classroom scenario one of the most difficult things is to first break down the students' existing misconceptions before actually teaching the material. 

That said:

From your post I gather you are based in Britain. One option for you is to pursue a degree outside the United Kingdom. The liberal arts type education in the United States, for example, give greater flexibility in the choice of coursework toward a degree program, and frequently you can make special placement arrangements to participate in higher/graduate level coursework in lieu of the introductory courses. Additionally, given you interest, maybe you should investigate degree programs in mathematical biology. Modern math bio tend to combine theoretical and computational mathematics (which involve a good deal of programming and simulations) with life sciences.