NHS prescription and dental care in the UK is free for women during pregnancy and till the time your child is one year old.
To claim this benefit, you will be issued with a maternity exemption certificate. You can ask your midwife about the procedure for obtaining one. Most surgeries ask you to fill in a form (FW8) and then a certificate is sent to your home address. Keep this certificate with you as some pharmacists insist on seeing it when dispensing medication.