I just found a discrepancy between UTM and UTMIP. I tried the following pair.
Lat: 56d 16' 35.46" N
Lon: 3d 23' 46.82" E
UTM Zone: 31
UTM coord: 6237064.70 N, 524543.14 E
It looks like the UTM seems working correctly, but UTMIP produced opposite sign for longitute coordinate. For this test, I used "International 1909" datum. Please let me know if I am missing something here. Thank you!