Some properties cannot have gas prepayment meters (ppm), it depends on who the gas shipper (company responsible for physically getting the gas into your house) is to your address. (you should be able to find this out by looking on your gas meter, it should say "property of .... " or some thing similar.
The gas prepayment system (or Quantum meter) is owned and operated exclusively by Transco, who ship to 95% of properties in the UK. If your gas is delivered by another smaller company, or IGT (independent gas transporter) such as IPL - Independent pipelines, SEPL - Southern Electric pipelines, Ironbridge, (plus more i cant remember off the top of my head) they do not have these meters and you simply cannot have one.
If they say they cant install a gas ppm, its most likely for that reason.
You should probably accept the elec ppm for the meantime and put all the money you can afford into paying off the gas. Contact them with money available to make a payment that day (I would recommend half at least) and they might allow you to make a payment plan for the rest, given that you cannot have a ppm.
You could try to contact Consumer Direct or the New Consumer Council (energywatch's replacements) but given the fact you haven't paid any bills, being honest, its a bit cheeky to complain about their collections procedures, you will have had a lot of reminders and letters before now about the balance of your accounts. I would only really contact them if EDF still do not accept your offer of substantial payment towards the balance of the account and are faced with the real threat of disconnection with nothing in your power to stop it.
That's not to say they will be either, but it might give EDF a "push" to arrange a payment plan with you.