I have recently moved into a new build property that has Openreach FTTP installed. I was attempting to order through a number of providers, some were apparently able to serve me whilst others were not. I contacted a number of the ISPs not able to serve me and they informed me that Openreach had yet to issue a UPRN for the address, so as a point of reference for new build properties this can also be an issue. The other ISPs who were able to offer me a service apparently did so using an alternative identifier and after selecting IDNet from amongst them I have had no issues with the delivery of service. It does appear that many CPs take their sweet time in updating their databases for availability even after Openreach have published the availability data!
As j0hn states, UPRN's are the domain of the local authorities, not OR.
We issue NAD keys (unique to each premises) - these are 'address matched' with the Post Office's data-base (well, they were up until literally a week or two ago, I think we're moving away from it now ??) ... if our info matches theirs, we assign a 'Gold key'.
If there's a disparity or a lack of info, we assign bronze or silver NAD keys as a temporary measure. I would imagine 'New builds' to be particularly susceptible to the latter two colours, especially if the Council address list has not been updated ??.
Not saying this is purely relevant to you, just giving an overview of where issues can lie.