It all sounds a bit of a mess OP.
As others have said, you don't have to take maternity leave - I went back to work 8 weeks after each baby because we couldn't afford for me to stay at home. We were only marginally better off after DD2, but it was worth it in the long run, because my salary increased over the years.
£2,700 sounds like a high monthly outgoing for two people. Do you know how this is split up? There may be things your partner is spending money on now that he intends to give up when you stop work.
What are the "very high" bills/credit committments" you talk about? Can you scrimp a bit while you're still earning and reduce some of these?
If you aren't entitled to working tax credits, then this may be because of your combined salaries being too high.
Your credit score wouldn't drop if you are regularly paying your mortgage (this would be in your favour) - are you already NOT paying regularly?
Could you list out what you jointly spend each month - either privately, or on here if you want feedback - to see where you could save?
You might like to go and see someone at the CAB - they will be able to help you review your finances and help you with consolidations etc