Hi voodoo I feel flattered that you might value my opinion. I am very aware of how 'militant' and 'up the revolution' I can be though, so ladies please remember these are my views and i completely understand why they might not be for everyone (my DSis thinks I'm quite bonkers btw!!). 
So, firstly I am angry on your part that your hospital have taken that attitude. How very dare they!! Assuming the only 'issue' is your BMI (which therefore IMO is not an issue at all) and there is no other medical reason specific to you, it must just be hospital policy that they are extremely risk averse.
To put your mind at rest, I recommend doing a bit of research on homebirths. Have a look at this site. It is a great starting point, especially the 'can I have a homebirth if...' link on the left.
Once you have some information about risks, you will probably feel more confident in standing up for yourself.
To be clear - No doctor or consultant in the world can tell you that you are not allowed to have a homebirth. It is not their decision to make. It is YOUR birth and no one can force you to go into hospital.
However, when it's on their territory they can refuse you entry to the MLU.
If you decide you want a homebirth, then simply write a letter to the head of midwifery at the hospital stating that you are due to deliver on xx date and that you intend to have a homebirth. Also state that you have researched your options and are making an informed decision to have a homebirth and you expect to be supported by midwives experienced in homebirth.
Keep it simple, don't be frightened by scare tactics and keep stating (if they push you) that you appreciate their comments but you intend to have a HB.
I recommend you have a look at the homebirth group on Yahoo groups. They are very supportive and populated by lots of independent midwives. link here
If after research you decide a hb is not for you, then they may decide the MLU is a better option if you keep going on about one IYSWIM! However, be aware that once in hospital, it is very easy for the consultants to recommend you move down to the labour unit, which is where they prefer you to be. Once in labour you are very vulnerable and it is very hard to stand up for what you want and, being human, the docs will try to persuade you by using statistics - not lies but also not facts that apply to you at the time.
You might want to consider a doula for your birth if you are keen to have a natural birth. She will be aware of your wants and stick up for you - asking the questions you may not think about and buying thinking time. Doulas cost about £200 www.doula.org.uk
Always ask - 'is this specific to me or is this hospital policy?' to make sure you get the best recommendations for YOU, not just because a doctor wants to tick a form.
Even in hospital remember, you do not have to consent to anything. You can refuse constant monitoring of the baby during labour (research shows it does not improve birth outcomes and can cause a higher rate of c-section). If I have to go into hospital to labour i will give them a choice - either no monitoring at all or intermittent monitoring but I do not consent to constant.
Sorry for the mammoth post - for my DD it all seemed so overwhelming, then I did my research and realised that the more interventions you have, the more likely things are to head towards a c-section. For example, most PPH (bleeding after birth) happens in hospital as it is generally caused by managed placenta delivery (caveat - not all, of course but alot). At home, it's generally recommended to have a natural 3rd stage and let the placenta come out by itself which reduces the risk of bleeding. Even if you do bleed, HB midwives carry equipment to stop the bleeding and then get you to hospital.
Gosh I have gone on. This is all my opinion based on my research. I am passionate about it but also do not want to bang on about it. Happy to discuss off thread if you PM me and keep smiling - it's your birth and your baby, no one else's!