OK, just to recap:
I live in a shit flat and I'm about to start a new job
You aren't able to move from your shit flat with one tiny damp bedroom because you can't afford it - but you might be able to afford a baby?
You're about to start a new job, so you have no protection against unfair dismissal for the first two years of your employment.
Have you checked your employment contract to see whether there is a length of service requirement for the 12 weeks full maternity pay? If so you will only be entitled to SMP at 90% of pay if you have worked for your employer for 26 weeks by the end of the 15th week before your baby is due.
In any case, you will not be able to afford to take more than the most basic time off, and go back to full time work.
At which point, you will either have to pay for childcare, or lose your DH's salary so that he can be a SAHD. Either way you will be much worse off than you are now.
After rent, bills, debts etc, DH and I have about £500 left over each month for everything else.
You have debts and no savings, and are currently unable to save anything from the £500 left over "for everything else" - or you wouldn't still be living in a "shit flat".
You currently walk to work and have no travel expenses, so what you gain in moving to a cheaper area is likely to be offset by the cost of commuting.
DH is subject to immigration control until 2015. His visa should be fine as long as I am earning more than £18k a year.
So if you lost your job, or became ill and unable to work between now and 2015, you would be in breach of your maintenance undertaking and your DH would have to go back to Brazil.
I'm tempted to ask my parents for a loan that is paid back once we're earning more
You're thinking you might take on more debt...
I think you need to sort your finances out first and foremost. Start earning more before you have your baby. Get the debts paid off, and get some money saved.
Then think about having a baby.
What's the worst that could happen?
You end up in emergency housing, on subsistence benefits. Your DH goes back to Brazil.