Hi, I did a foundation year at Manchester last year. There's no definitive answer to your question as 'easy' is very subjective, but I can elaborate on some details based on your questions so you can hopefully get a better idea of what to expect. First of all, the pass rate for modules vary across different courses, and there isn't just a single number to look out for. How foundation year works is that you'll likely have around 6 modules per semester, so 12 in total for the whole year. These 12 modules will make up 4 'superunits' which are each comprised of three modules categorised by subject materials. For example, last year I had 2 maths superunits (comprised of six math modules), 1 physics superunit (3 physics modules), and a project superunit. Each of these superunits will have their own pass requirements. Chemical Engineering had the highest passing requirements last year for Engineering (around 70%), so it depends on what course you're planning to take after your foundation year. Don't worry if this sounds confusing, as if you do end up taking the course, your lecturers will explain this system in detail.
In terms of the subject material, Physics is very similar to A-Level Physics which is quite a jump from GCSEs. The Maths modules are a mix of A-Level Maths, A-Level Further Maths, and some degree level maths, which is also a rather big jump from GCSEs, but since you did take GCSE Further Maths you may be familiar with some of the early content. I wouldn't say you're severely disadvantaged as a lot of people in Foundation year did not take A-Levels or IB. The good news is, the uni does take this into account! At least for maths, the maths modules for people who didn't take A-Levels and people who did take A-Levels are different, so you won't feel like you're scurrying to catch up with everybody else. If you put in the work and are consistent with your revision, it's very much a manageable course. Don't be hesitant to ask for help from your peers, lecturers, or tutors as they are all very helpful!
If you'd like some more info on foundation at Manchester feel free to ask