At university level, universities don't normally use letter grades. Instead, they refer to percentages / classifications (1st, 2:1, 2:2, 3rd, as detailed above). A few universities have started using letter grades, but I'm not aware that any department at Manchester has done so, and it seems to be causing confusion at those universities anyway. However, it appears that foundation courses are graded very differently to degrees, and so I can't really help there!
Foundation courses are designed to bring everyone up to the same standard as people who have done A Levels and got grades good enough to go straight into the first year of the BSc. They will be quite used to people who are fundamentally intelligent, but for whatever reason didn't do well enough in their A Levels, did A Levels in inappropriate subjects (Drama, Music and English for a Pharmacy degree, for instance) or are from foreign schools where they didn't have qualifications suitable for direct entry into the BSc.
Once you've done the foundation year, you'll be in a really good position to start the degree, so I think you'll be fine
Given that the Xaverian foundation course is more focused on life sciences, I think that that would be better for you. INTO is likely to have a lot of ESOL stuff in, which isn't going to help you at all. In addition, it would be a big risk to go for INTO, because there's no guarantee that you would actually get an offer.