The law required that the birth be registered within 42 days. This link has all the information on the registering process, I hope it helps you.
http://www.babycentre.co.uk/pregnancy/naming/registerbirth/

42 days is the limit for registering the birth but the fathers name can be left blank if it is "impossible, impractical or unreasonable" to identify the baby’s father. In this case, the father’s name can be added any time after the registration.
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You need to complete a GRO 185 form. Both parents are then required to attend the registry office and register the birth again in the presence of the registrar. The father's details will be entered and the baby's name can be changed to the father's or a double-barrelled name. Once this is completed, the new birth certificate will be issued and a fee of £3.50 is charged. http://212.248.225.145:8080/publications/GRO185.pdf

If either parent is not able to attend or is not even in the country it is possible for that parent to make a statutory declaration of parentage before a solicitor or similar.

If either parent is not willing to co-operate in having the father's name inserted into the birth registration the other parent can seek a Court Order. A certified copy of the Court Order will have to be submitted to the Registrar General's Office and they will authorise the re-registration of the birth by the parent who has obtained the Order.

http://www.brent.gov.uk/faqs.nsf/0/45ae616136deeb0b802568d900578084!OpenDocument