There's two questions there really - can they? Absolutely yes it's an option for them. Will they? Highly unlikely if you engage with them, explain your circumstances and if possible come to an arrangement to pay.
Forcing prepay is a last resort for them, and Ofgem introduced new rules last year that make it more difficult/expensive for them as an option.
I can't emphasise enough that you should speak to them and keep in contact with them.
They won't expect you to clear the debt immediately. If they do then complain.
Things to consider and information to ask for them:
Can you afford to pay for what you're using day to day? They will be able to work out what you use so make sure they tell you that.
Do you have functioning smart meters? If not, read your meters before you speak to them so the amount you owe can be calculated accurately.
Have details of your income and outgoings handy so they can complete an affordability assessment with you.
Be really clear about the impact of your MH issues and physical health issues. For instance, do you have medicine you keep in the fridge? Do your physical health conditions get worse if you're cold? How do your MH difficulties affect your ability to manage your day to day affairs?
Don't be pushed into a plan you can't reasonably afford, but also be realistic about paying something. Don't be afraid to say that you want time to consider it, or that you would rather go and speak to citizens advice (or another such organisation ) first and get advice.