May be held by LA planning dept as part of a planning/building regs application; by a landlord on their property management system; or even as part of an estate agent's particulars if there's been a recent sale of the property. (Note that conveyancing is also using the UPRN as its standard identifier.) Housing developments may well have a number of homes built to a standard floor plan.

Need to be aware of possible adaptations/modifications; plans can't be guaranteed to be accurate or up-to-date.

If available (and accurate!), could be useful to an OT or to a fire service attending a callout.