As @Tiredtiredtired100 says, you need to claim your SMP from your previous employer. If you were with them for 6 years up until 35 weeks pregnant you would have been eligible for SMP from them, regardless of redundancy. You need to contact them and call ACAS for advice.
In the mean time, you will get child benefit, if you applied soon after the birth you should hear back very soon. Have you checked if you qualify for universal credit? Call BT again and say you need to terminate the contract, rather than asking to reduce, they usually come up with some way they can cut the bill then.
As you need money ASAP, rather than being able to wait for contracts to end/car insurance renewal, I would contact your IVA provider and see about either a payment holiday (max 9 months break from payment, which would cover you being on maternity) or reducing payments. That will help you out more immediately.
If you need to cover food for yourselves the contact your HV, they should be able to give you food bank vouchers, you can also speak to them if you need formula for the baby. Don’t be worried about approaching them, being made redundant wasn’t your fault and it has obviously affected your circumstances.