I got my prescription exemption after my booking appointment (so by 10wks). MW set it up for me:
"If you are pregnant, or have had a baby in the last 12 months, you get free:
NHS prescriptions, but only if you have a valid maternity exemption certificate (MatEx)
NHS dental treatment if, when you are accepted for a course of treatment, you are pregnant or have had a baby in the previous 12 months
To apply for your maternity exemption certificate (MatEx) ask your doctor, nurse, midwife or health visitor for form FW8. You complete parts 1 and 2 of the form and your doctor, midwife or health visitor signs it to confirm that the information given by you is correct.
Your MatEx will last until 12 months after the expected date of birth of your baby. If your baby is born early you can continue to use your MatEx until the certificate expires. If your baby is born late you can apply for an extension. If you apply after your baby is born, your MatEx will run for 12 months from your baby?s birth.
For more information on how to apply for a MatEx visit the NHS Business Services Authority website. "
Just ring your MW and ask by the looks...