changing you child's name on their birth certificate has to be done under deed poll and can only be done by the person with parental responisbility. If an estranged/seperated parent also has parental responsibility, then they have to give their consent to the change also. It has to be done via a solicitor. The child then will always be known as her/his new name. When applying for passports/banks etc, whenever the birthcert has to be shown, the certificate of deed poll accompanies it. After a passport is recieved, it is sometimes easier to use your passport instead of your birth cert when the occasion requires it.
You can add your husband's name to the birth cert. Just go to the registry office and ask for the application form. If you get no joy, ask to speak to a more senior registrar.
good luck and it is lovely that you have ironed out your probs...Congrats on the pending marriage also. :-)