I'm not sure the government help pay for childcare for people who are ill off work tbh, but have a look at entitledto website in case. How bad is your illness, really that bad that you can't look after your child?
You'll get your annual leave entitlement for as long as you are employed, so as long as you don't resign, the fact that you are off sick during your pregnancy will make no difference.
In terms of your maternity pay, if you are entitled to more than statutory, then your employer sets the rules about that, so read your maternity policy to check.
For statutory maternity pay, it's all based around whether you've been employed with the same employer for 26 weeks by 15 weeks before your due date, and on your average earnings in the weeks 17 - 25 of your pregnancy.
Assuming you meet the length of service criteria, the problem will be if you are/have been off sick during weeks 17-25, you will probably not have been earning enough to entitle you to SMP.
If you don't get SMP, you will probably get Maternity Allowance, which isn't from your employer, it's from the government, and is similar to SMP, but on basic weekly rate throughout, ie without the first 6 weeks at 90% of salary.