It's not helpfully laid out, but if you can make sense of the links that Snufkin has provided, then there are differences in the available module options at levels 2 and 3.
The common modules for both degrees that I assume you would take are S206 Environmental Science (60 credits), S396 Ecosystems (30 credits) and SXE390 Environmental Science Practical Project (30 credits). All of these follow on nicely from each other, and feature equations to explain how the environment works, as well as the statistical tests used to make sense of environmental data, which are introduced in S206, examined further in S396 and you would use them for your own data in SXE390.
If you are wanting to avoid the higher level maths then Environmental Science looks like the way to go, unless you did a Natural Sciences pathway focusing on Earth Sciences, Environmental Sciences and a bit of Biology or Health Sciences. The Physics, Astronomy and Chemistry modules are where all the trickier maths comes into play. Having said that, there isn't any calculus even at level 3, unless you do the quantum physics or relativity modules.
If Earth Sciences and Environmental Sciences are what float your boat, then you would want to be finishing your degree with either SXE390 or SXG390. SXG390 is more of a literature review of many academic papers around a chosen geoscience topic, so it is not advisable unless you take S209 and probably another geoscience module at level 3. SXE390 would possibly be better even though you have to do some maths (statistics) on your results - it all gets taught in S206. Also, if you did an Environmental Sciences degree you would get to take U316 The Environmental Web for a whole 60 credits at level 3 which has almost no maths in and is more about teaching how ecosystems/environment are linked to culture and society and industry and all that.
If you stick with the maths, you will be surprised how much you get out of it. I was terrified of the maths involved in a sciencey degree before I started with the OU, but I wish I'd taken more quantitative modules now that I'm nearly finished!
I hope all the above isn't too confusing, feel free to ask more specific questions