If when purchasing a title document you see the message
"The title number provided is a pending first registration so we are unable to supply an official copy at this time. Please make a new request when the first registration is completed"
this means the title is on the process of being registered with the Land Registry and isn't available to be purchased yet.
It can take up to 12 weeks for the Land Registry to complete the first registration process and make the documents available to the public.