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Keep 1 or 2 as historic items, replace the rest
In this case i think they'll keep all of them but replace the gas mantels with retrofit modern LED that's been packaged to look authentical. Given the length of time they've survived they've been great in terms of waste materials (i get asked to look at redoing a lot of lighting schemes that are less than 10 years old with no decent recycling policies). But yes offsetting the energy usage isn't sustainable.
One thing to consider is how wonderfully pants gas lanterns are in terms of output and anything LED is going to be a lot brighter. The city is already way too bright and too many new schemes fall painfully short of addressing this.
Slow day at work? If so i'm interested to hear people's take on this
https://www.theguardian.com/uk-news/2021/nov/21/plan-to-change-westminsters-historic-gas-street-lights-to-leds-sparks-anger?CMP=Share_iOSApp_Other
Get rid or save our blessed country's history?
Through work i'm familiar with both sides of the coin, promoting low energy more sustainable lighting and also sometimes working with heritage to protect older stuff. Personally i'd like to see them saved as offsetting 300 gas columns shouldn't be difficult if Westminster sorted some of the many other crappy lighting schemes they manage.
Of course the best way to save them would be to tweet "forrinerrs trying to steal our queens light posts" and then watch UK's finest arrive to defend them.