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[Worf]

The UK regs are vastly out of date (1986??) and take no account for modern "improvements"
Still using watts for brightness (should be lumens?)
However, I just read a bit of the regs and apparently an indicator bulb can be up to 38 watts

That seems pretty bright, maybe to cover HGV's 

You learn something every day.

[O7BE16]

Direction Indicators for new vehicles are covered by UNECE regulation 6.02 which was last updated in 2019 which sets limits for light intensity in Candelas (cd)

[Wittsend]

Good luck in finding a 38 watt bulb....

candela
The candela (symbol: cd) is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function.

 

Candela

From Wikipedia, the free encyclopedia
This article is about the unit of luminous intensity. For other uses, see Candela (disambiguation).
candela

Photopic (black) and scotopic (green) luminosity functions. The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data). The horizontal axis is wavelength in nm.
General information
Unit system   SI
Unit of   luminous intensity
Symbol   cd
Conversions
1 cd in ...   ... is equal to ...
   international candles      ≈ 1.02 cp 
   Hefner Kerze      ≈ 1.11 HK 
The candela (/kænˈdɛlə/ or /kænˈdiːlə/; symbol: cd) is the unit of luminous intensity in the International System of Units (SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the standard luminosity function (a model of the sensitivity of the human eye to different wavelengths).
A common wax candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured.

The word candela is Latin for candle. The old name "candle" is still sometimes used, as in foot-candle and the modern definition of candlepower.

Definition
The 26th General Conference on Weights and Measures (CGPM) redefined the candela in 2018. The new definition, which took effect on 20 May 2019, is:

The candela [...] is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W−1, which is equal to cd sr W−1, or cd sr kg−1 m−2 s3, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.

etc.
etc.