Hi there 2ys. First and foremost congratulations to you both .My first daughter was unexpected and is wonderful now 30 years old and a mum herself. I have not one regretand it was hard I was young but so worth it.
First person to speak to is your IP and discuss your options.There will be options, possibly a variation to reduce payments, remember I and E will change with a babe,is bankrupcy an option for you, a shame after going three years, but still an option. Your IP will be able to give you the best advice. So far as letting agents I have seen on the forum it can be harder, but not impossible to rent somewhere. Be upfront tell them the situation at the beginning and they will be able to tell you how the land lies.
I am not a technical expert, and this is only my opinion, but this is much more important than money, if you are both happy, it may not be when you would have wanted to become parents, but it doesnt mean you wont manage.
Speak to your IP, look at housing associations, speak to them, speak to to the letting agents.
Hope that helps a bit.
All the very best to you both .
Ang