Richtwert: benchmark, Zahn bench mark, Dietl/ Lorenz standard (basic) value, Herbst.

Regional : also 'Provincial' for Austria vs. Federal State in Germany and Cantonal in CH,

Osterr. RechtsWB : vom Bundesminister für Justiz für jedes Bundesland festgesetzter Schillingbetrag je Quadratmeter der Nutzfläche und Monat für die mietrechtliche Normwohnung (§1 RichtWG; § 16 MRG / Mietrechtsgesetz). -> 'The Richtwertmietzins defines a certain standard rent value per square meter. This value differs in every Austrian state and is redefined every few years.'

Not sure of the UK equivalent, though within the four corners of the UK Rent Protection - now Housing - Acts.

I would firstly retain the German and add an explanation in square brackets
As far as I can tell, the rent legislation in Germany is highly complex
I was tempted by "benchmark" and even "fair rents" and "standard rent" (if there is even such a thing in these fluid times) but I think at the end of the day "rent control" is what it is all about

Does Germany have rent control?
Rent control has been in force in many of Germany's largest cities, including Hamburg, Berlin, Stuttgart and Munich, for many years. This stipulates that “cold” rents (excluding utilities) cannot be more than 10 percent higher than the local comparative rent.30 Mar 2022

What is index rent in Germany?
According to § 557b BGB, the index rent stipulates that the rent is linked to the consumer price index. The consumer price index is determined by the German Federal Statistical Office and is based on the cost of living of all private households in Germany.

How much can I increase rent in Germany?
Legally, your landlord cannot increase your rent within the first 12 months of your tenancy contract, or by more than 20% (15% in some federal states) over a three-year period. Some landlords, therefore, charge “stepped rent” (Staffelmiete), that increases gradually over time.