1. Jersey is in class A - correct? But if I am an IMCr holder, I can go IFR down to 3km vis?
Jersey is actually in class D. But you can't get to the class D without passing through the Class A zone around it.
As an IMCr holder, you may fly only under special VFR within class A (CTRs), not under IFR. You may do so in a visibility of 3km or more.
2. That being the case, do I call Jersey up asap and ask for IFR clearance to the ILS when I'm crossing the UK coast (assuming I've filed a FP)?
No, you call Jersey Zone and ask for a SVFR entry clearance.
3. Or, do I ask for VFR clearance and vectors to the ILS?
Nope, no VFR, and no vectors to the ILS. SVFR only.
Procedures are in the
AIP in EGJJ AD 2.22 .