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You should make passport enquiry application at the British embassy in Pakistan. Your child is a British citizen by law so no registration is required but you will be asked to provide evidence that you are British otherwise by decent.
You would need to contact the British embassy in Pakistan given that your child resides in Pakistan and ask if they accept British passport applications failing that you may contact Her Majesty's Passport Office. They may very well interview you. Yes it will usually involve downloading and filling the relevant forms and there may be an interview.