Hi Miffster, I do get it about London traffic - and our hospital is in central Reading (horrendous traffic) and along the M4 from us (can be major delays).
Have you asked your MW what the blue-light transfer times are? They're usually far less than driving yourself or getting a cab as you run red lights, etc.
If you're not happy going there under any circs, you have the choice of either not having a home birth or hiring an independent MW.
You could try asking if your MW would book you in for both hospitals. It's not really allowed but I know my MW was happy to do that for a while at least when I couldn't make up my mind. Not sure you could keep that going till birth though.
PCT boundaries really confuse people in London as they're all so close anyway, but that's just the way the NHS is set up. If you want the NHS to supply CMWs for a home birth, that will be paid for by whatever PCT you live in, so care will be linked to the PCT hospital.
In a real emergency, an ambulance will take you to the nearest hospital, but if you don't get to stay home for some reason, or want to transfer for other reasons (pain relief, slow labour) then of course you go to the hospital you're booked in with.
As I see it, you've got 3 choices (if your MW won't bend/break the rules to book you into 2 places):
1, stay booked in for a home birth with Homerton as back-up (with or without doula).
2, book into the Whittingdon MLU.
3, book into the Whittingdon and hire an independent MW for a home birth.