Their certificate reads: 1864 Marriage solemized 'at the Catholic Chapel Broad Street'' in the 'District' of 'Hereford' in the Count'ies' of 'Hereford and Monmouth'
I had presumed as the Broad Street chapel had no been recognised as a ‘proper’ church at this time?
It depends on what you mean by a "proper" church and who was using the words "church" or "chapel" and the context.
St Francis Xavier Church was opened 1839. A representative of Queen Victoria attended the opening ceremony, the first time a royal representative had been present at the opening of a Catholic since the Reformation. It was definitely a church not a chapel. There may be newspaper reports.
A wedding ceremony in a Catholic church in England had no legal validity unless a registrar was present. The registrar was in charge of filling in the civil register. Standard wording on a marriage certificate would have been policy or custom of the registry office. Hereford District and perhaps other districts in Herefordshire may have called every Catholic church a chapel, in line with the dictionary definition of a chapel as a place of worship not of the Established Church, i.e. not Church of England.
It's still common in Scotland to refer to a Catholic church as "the chapel". My local Catholic church is almost 200 years old and is now the cathedral for the diocese but some non-Catholics would call it "the chapel". The term distinguishes it from "kirk" (meaning church), the place of worship of the majority Christian denomination, Church of Scotland, which is Presbyterian and from "church", the place of worship of the Episcopal Church in Scotland, which is Anglican.
Discrimination laws against minority religious denominations were gradually relaxed in England. There was a licensing system for their places of worship. The Second Catholic Relief Act 1791 permitted public places of worship and schools, under certain conditions. A Catholic place of worship had to be registered at Quarter Sessions, wasn't allowed a steeple or bell and doors had to be left unlocked during services. Surviving licences may be in county archives. Applications may have been in the name of the clergyman or the owner of the property.
"Returns of Roman Catholic Chapels registered 1791-1852" National Archives RG31
1836 "Return of Dissenting Meeting Houses and Roman Catholic Chapels in England and Wales" [Parliamentary Paper]. gives meeting houses and chapels registered up to that year.
The religious census 1851 included R.C. and Protestant nonconformist churches and chapels. It gives attendance at services on 30th March 1851 + information on buildings. National Archives HO129. Some local record offices have a copy of returns in their county.
1852 Act of Parliament transferred registration of places of worship to Registrar General.
Website "Researching Historic Buildings in the British Isles" has articles on ecclesiastical buildings with timelines, source lists and links to further information.
https://www.buildinghistory.orgRelevant topics include: "Nonconformists", "Nonconformist churches and chapels", "Survey and valuation of ecclesiastical property".
National Archives guide to Catholic research
https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/catholics/A standard work is "Catholic Missions and Registers 1770-1880" (6 volumes) by Michael Gandy.
The Catholic Record Society published transcriptions of many historic Catholic registers a century ago. Introductions to each transcription often include history of the mission and churches.
You could look at census returns for Moor Street in 1851 & 1861 to see if there were any residents whose occupation was religious or educational.