4000 lines of mostly comments and printf statements along with a 2kb coefficient file.
You only have 2k of data space in the uno so no chance. But the mega should be able to do this. You will need to strip out all the printfs and some /all the includes. You can't use stdio.h for example.
For the UK the difference between true and magnetic north is give on the OS maps and does not change over the 3/5 years you want.
Mark