The source wrote, in part (emphasis ours) (citation):

The sole source of data for HHPI (2019) remains Lloyd's Banking Group (LBG). [...]

With the core aim of HHPI (2019) to provide a robust measure of changes in residential property prices, as in the case of HHPI (1983), several types of transaction are excluded from index generation. These include:

• Re-mortgages
• Business use, capital raising, or building mortgages
• Discounted mortgages relative to market value (as determined by observed data that shows the property valuation < 75% of purchase price e.g. the “Right-to-Buy” scheme).

An exception to the discounted mortgage rule is shared ownership mortgages which are, for the first time, included in the new version of the HHPI. We also now include buy-to-let properties and those bought directly from less-conventional vendors e.g. builders.

The new methodology also excludes variables such as central heating statistics and garage spaces.

In response, we have:

1. Archived the predecessors ("HHPI (1983)", 1983=100) by renaming with an "_83" specifier, marking the metadata as "discontinued", and segregating in the catalog.
2. Created one-for-one replacements ("HHPI (2019)", 1992=100).
3. Refurbished the catalog.

Properties that have changed:

• Index base period: from 1983 to 1992
• Start date: from 1983m1 to 2018m9

The series reside in the historical catalog:

• United Kingdom » Real Estate » Residential » Prices » Halifax House Price Index » Discontinued » HHPI (1983)
• United Kingdom » Real Estate » Residential » Prices » Halifax House Price Index » HHPI (2019)

and are:

• HSIHPH_83AM.IGBR = [DISCONTINUED] Index, (Index 1983=100, SA)
• HSLHPH_83AM.IGBR = [DISCONTINUED] Standardized average price, (GBP, SA)
• HS%HPH_83UM.IGBR = [DISCONTINUED] Annual percent change, (% Y/Y 3-mo MA, NSA)
• HSIHPHAM.IGBR    = Index 1992=100
• HSLHPHAM.IGBR    = Standardized average price
• HS%HPHUM.IGBR    = Annual percent change

References

• Halifax House Price Index - home page
• Methodology