The homebirth website linked to is very informative. Other sources you might find useful include:
homebirthUK mailgroup
AIMS
It is not too late for you to change hospital if you want to do so. It might be awkward and cause more paperwork for the MW's, but if you no longer have confidence in the team with the hospital where you are booked and would feel safer and better supported if you changed then go for it.
The legal position is that the only way you can forced to attend hospital whilst in labour (or to accept any intervention) is if you are sectioned under the Mental Health Act. As a mentally competent adult the hospital do not have the authority to "allow" or "let" you do anything - although they often use language that makes it appear that they think they can.
Most PCT's have a policy of only supporting a homebirth between 37 and 42 weeks - some restrict that further (say 38-41). The logic behind that as I understand it is that anywhere between 37 and 42 weeks is considered "term" - a baby that is born pre-term is more likely to need a level of support that cannot be/is not provided in a home environment. Once you get into post-dates territory there are also issues that become more common that could be dealt with more effectively in hospital. BUT even then - you cannot be forced into hospital. You are not their employee. You are not bound by their protocols or policies. They can advise. You are not bound to accept that advice. That said, the further outside of the norm you take the parameters of your HB the less comfortable and confident the MWs will be. IMO I'm not sure that's fair on anyone involved.
Whether or not MWs have a legal obligation to support you wherever and whenever is a bit of a gray issue. They have a duty of care - if you call and say you are in labour and ask for help they have a duty of care to supply that help. But the labour ward might choose to supply that help by sending an ambulance. Or the labour ward might, genuinely, have no MWs to send so have no choice but to send an ambulance. You are under no legal obligation to get into that ambulance, of course, and anecdote says that women (or their birth partners) who are prepared to repeat "I am in labour. Please send me a MW. I will not come into hospital. I will not allow paramedics into my home" or words to that effect do get a MW.
re SD... Yes, 50% of cases happen in babies that aren't macrocosmic (I think you might have a typo - DD was over 4kg and she was big enough. I REALLY don't want a 9kg newborn!). However, less than 50% of babies will be macrocosmic, so SD will be more frequent in larger babies. That still doesn't mean its common - and IIRC the NICE guidelines seemed to say that it couldn't be predicted accurately enough to justify changing care plans to try and avoid SD (assuming no previous occurrence). I'm not trying to deny that SD is a genuine obstetric emergency and there are things that can only be done in hospital (both in terms of emergency birth, although rarely, and in terms of supporting the baby afterwards if required) - but that's very different to saying that nothing can be done at home. SD at home is not destined to end in a bad outcome any more than SD in hospital is guaranteed to have a happy ending.
Has the Dr managed to explain why a large abdomen (which is squishy) makes shoulder dystocia (problem with bony things) more likely? Have any of the other measurements on your baby been large?
Where to go from here - first off decide whether to change hospitals or stay put. Have you contacted the hospital you'd like to change to?
Assuming you're still not being supported in your plans for a HB, write to the Head of Midwifery stating that you have been advised of the risks related to SD but that you have also been advised that, given the short duration of your first labour, this labour may be too short to allow you to get to hospital safely. As such, you have decided that a homebirth is the best option for you to birth this baby safely and you expect to be supported in you plans for the birth of this baby in your home. You might want to ask her to inform her MWs that this is your decision such that your antenatal appointments can cover any issues relating to HB that need to be discussed in the next few weeks. As you are well into the third trimester you anticipate her reply within however many days you think reasonable. Send it by a method such that you will have proof its arrived. If you join the mail group or talk to AIMS I'm sure someone will be happy to supply you with a more detailed template to use :).
So basically, yes - just tell them you intend to stay at home and let them deal with it :o