I found this chart of holy days of obligation in UK. It doesn’t seem to have Immaculate Conception . Maybe it can help someone with respect to the other holidays.
I agree it is confusing when you are in a different country, or even when in the US, in a different diocese from one’s normal one as sometimes the dioceses make different rules.
I myself would want to just go anyway, though it’s not obligatory, but if that’s not possible for you, maybe because there aren’t Masses at a time when you can attend, then I understand.