Unique Street Reference Number is a unique number given to each street in the National Street Gazetteer defined in New Roads and Street Works Act 1991 and also BS 7666. Is there any value in recording this data in OSM?
I am not familiar with tags, but I think it would need to include a country code idenfitier (e.g. “GB”) and a code to identify the numbering type (e.g. “USRN”). This would give something like “???USRN=123456”. There should also be metadata about the metadata to describe when this metadata is valid from and to (e.g. from:2012-01-01 to:2015-12-31).
As you may by now have noticed from the posting dates, this forum is largely ignored by users in the United Kingdom (they tend to communicate by one of the help site, the talk-gb mailing list and the #osm-gb IRC channel). You’re probably more likely to get a reply in one of those places.